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Key Concepts

Here we sort out some of the key concepts, history and background in infectious diseases.

Antibiotic resistance


Disease elimination vs. eradication

False balance

Incidence vs. prevalence

One Health

Pandemic influenza

Quarantine and isolation



Antibiotic resistance

First introduced to the public in 1944, antibiotics – drugs that kill harmful bacteria – have all but eliminated the threat of diseases that once killed millions including sepsis, tuberculosis, plague, and cholera. But overuse of these drugs in people and livestock animal farming has resulted in the breeding of “superbugs,” or germs that are resistant to most or all existing antibiotics.

Bacteria become resistant to antibiotics naturally. When an antibiotic is used, most bacteria are eliminated. A few resistant strains survive, however and continue to multiply and spread. Some of these bacteria are resistant due to a genetic mutation, or because it acquired resistance from other bacteria. Mutations, rare spontaneous changes in bacteria’s genetic material, enable bacteria to inactivate or close off from an antibiotic. Bacteria can also acquire resistance by “mating” with one another, transferring genetic material with antibiotic resistance. Bacteria can collect multiple resistance traits, making it resistant to many if not all antibiotics.

Until the early 1980’s, pharmaceutical companies invested in developing new antibiotics to keep ahead of evolving resistance, but the scientific and economic challenges of developing new antibiotics has led to a steep drop off in the availability of new types of antibiotics, even as superbugs have spread. In the 1950’s, drug companies introduced at least nine new types of antibiotics. Since 1984, there have been no new registered classes of antibiotics.

Most antibiotic resistance cases have been associated with healthcare settings such as hospitals and nursing homes. Increasingly, however, there are cases of antibiotic-resistance – methicillin-resistant staphylococcus aureus (MRSA) infections or drug-resistant tuberculosis, for example – outside health care settings. These cases are hard to treat with existing antibiotics.

At least 2 million people in the U.S. contract an antibiotic-resistance bacterium annually and 23,000 die as a result. The threat has become so dire that in 2013, the Centers for Disease Control and Prevention (CDC), announced that the globe had reached a “post-antibiotic” world.

A British report, The Review on Antimicrobial Resistance, estimates that as many as 700,000 people die each year around the world from infections that can’t be killed by antibiotics. The same report estimated that if nothing is done to stem the growing number of antibiotic-resistant bugs, as many as 10 million people could lose their lives annually by 2050.

Public health leaders have been working on strategies for reversing the trend. The CDC is pushing physicians to stop overprescribing antibiotics and farmers to stop using antibiotics in raising livestock. A large percent of antibiotics are prescribed for farm animals, which many experts believe is accelerating the spread of antibiotic resistance. For example, colistin, an antibiotic of last resort, was widely used in animal farming in China. Then colistin-resistant bacteria showed up in some patients, most recently in the U.S. when a Pennsylvania woman was found with a colistin-resistant bacteria that caused a urinary tract infection. China has since banned the use of colistin in animal farming.

To spur drug company investment, Congress in 2012 passed the Generating Antibiotic Incentives Now (GAIN) Act as part of a reauthorization of funding for the Food and Drug Administration, which grants longer market exclusivity for antibiotics and streamlines the process for regulatory approval of new antibiotics.


Bioterrorism, the act of turning biological agents like microbes or toxins into weapons, has been used by military leaders for more than 2.000 years. One of the first known uses of bioterrorism occurred in 184 B.C., when Hannibal, the leader of Carthage (modern-day Tunisia), directed his sailors to fill earthen pots with serpents and launch them at enemy ships led by King Eumenes of Pergamum (Turkey).

During the 1960s, the U.S. military had a biological arsenal that included numerous weaponized pathogens, as did Canada, France, Britain, and the Soviet Union. But concerns about risks of such programs to society led to a 1972 U.N. Convention prohibiting the development, production and stockpiling of infectious diseases. The agreement was signed by 103 countries, and the U.S. confiscated its arsenal of bioweapons. But the convention had no enforcement mechanism. The Soviet Union, for example, kept working on bioweapons, through a program it called Biopreparat. Not until the late 1990s, when the Soviet Union dissolved, did the U.S. learn of the program.

The U.S. helped to dismantle the Biopreparat program, but security experts believe some countries, like North Korea, are still working on bioterrorism. Further, terrorists no longer need the backing or the resources of a nation-state to build a bioterrorist weapon. With developments in technology, only determination and access to medical supplies and a laboratory are needed, as the U.S. learned in the 2001 anthrax attack.

Anthrax isn’t communicable between people, but the bacterium can be altered to easily spread through the air. When inhaled, anthrax can cause lung damage and death. Not long after the Sept. 11 terror attacks, someone altered anthrax spores and sent them to the media and members of Congress through the mail.  After a years-long investigation, the Federal Bureau of Investigation declared that Bruce Ivans, an Army microbiologist, was responsible for sending the spores, which infected twenty-two people and killed five. Ivans killed himself before he was charged and doubts remain about whether he was the culprit.

The anthrax attack remains the worst bioterrorism attack in U.S. history, and national security experts worry another one could occur.

Since 2001, the government has spent billions on programs to respond to a potential biological attack. One of the programs, BioShield, directs the federal government to stockpile medical countermeasures to respond to a chemical, biological or nuclear attack. Another program is the Biomedical Advanced Research and Development Authority (BARDA), which operates within the Department of Health and Human Services’ Office of Preparedness and Response. Among other measures, BARDA has developed anthrax drugs and diagnostic tools.

Despite all the money that has been spent, biodefense has been handled by various pieces of the federal government, and national security experts have been concerned that the U.S. isn’t positioned to effectively respond to a bioterrorist attack. Currently, there isn’t a single coordinated inter-agency strategy for responding to a bioterrorist attack or a naturally occurring pandemic. 

To change that, Congress at the end of 2016 directed, the departments of Agriculture, Defense, Health and Human Services, and Homeland Security to work together on a national biodefense strategy. Though there were concerns among security experts that the Trump administration didn’t have a team in place to produce such a strategy, Tim Ziemer, White House National Security Council senior director for global health security and biothreats, in November 2017, told a non-profit security group, the Blue Ribbon Study Panel on Biodefense, that the Trump administration would be releasing a biodefense strategy by the end of 2017 or early in 2018. As of January 2018, a strategy had yet to be released.

Disease elimination vs. eradication

The elimination of rubella in the Americas was announced in April 2015, followed by the elimination of measles across the Americas continents in September 2016. Yet measles cases still occur in North and South America, and news is still being reported on the eradication of polio, which has not been seen in the Americas in years. The key here is that “elimination” and “eradication” are different things, though they are often confused by readers and sometimes even by journalists.

Eradication refers to a disease being completely, literally eradicated from the earth: no cases occur at all, from any source. The best-known example is the eradication of smallpox in 1980. Another lesser known disease that has been eradicated includes the livestock virus rinderpest. Campaigns to eradicate polio and Guinea worm are officially underway, and it could be argued that public health officials are — so far unofficially — working toward eradication of hookworm, measles, rubella, malaria and other diseases.  Only certain conditions are able to be eradicated with current tools. An example would be a disease which lives in the environment, rather than requiring a human vector.

Elimination, however, refers to a permanent interruption in indigenous transmission of a disease, making it no longer endemic, but the disease can still be introduced by a case from another geographical region. Or, as it was put in an article about the measles elimination, “Measles no longer lives in the Americas though it occasionally visits.” For example, measles has been eliminated from the U.S. since 2000, but there have been a number of measles outbreaks in the U.S. since then. All of those outbreaks, however, were introduced by a person visiting from outside the U.S. None of them began with a person already living in the U.S. because the virus no longer circulates on its own in the U.S., thanks to the effectiveness of the measles vaccine.

The distinction is important because an eliminated disease can always return if conditions allow for it, such as a sufficiently deep, sustained drop in immunization rates that allows measles to begin circulating again.

False balance

This lapse in responsible reporting refers to using outliers’ voices to state opinions that contradict the facts simply to provide “balance” to a story. Stories about any topic certainly need to include as many perspectives on an issue as possible as long as those perspectives are purely opinion-based (something that science cannot show to be true or untrue either way) or those perspectives are supported by some scientific evidence, even if that evidence diverges from other evidence. However, if such a strong consensus from the evidence exists that something is regarded as a fact, then including a person who doesn’t believe that fact does not provide accurate or appropriate balance to a story — it just confuses the reader about what the facts are. A flip example would be including a quote from someone who believes the earth is flat in a story related to weather or the curvature of the earth, or quoting someone who believes the moon landing was a hoax in a memorial story about the moonwalk. In reporting on medical research, this becomes tricky because scientists are learning more information all the time, and it’s reasonable for journalists to seek countering opinions particularly on new research, such as new findings about the gut microbiome or a new treatment. Other topics, such as breast cancer screening, may have contradictory evidence or involve controversial opinions on what to do about the evidence, all of which should be considered for a story.

One of the most common examples of a topic that falls prey to false balance, or false equivalency, in reporting is vaccines, mostly in smaller markets or by general assignment reporters who are less familiar with the health or science beat. The way the media’s falsely balanced vaccine reporting damaged public health reporting (and consequently public health) is such a well-worn case study that CJR featured outstanding coverage of it in Curtis Brainard’s Sticking with the Truth. Quoting “both sides” on concerns about a safety issue in vaccines that has been demonstrably shown not to exist makes it appear that there is a controversy among experts when there is not. The group Voices for Vaccines offers an excellent primer to false balance and how to avoid it in accurate news stories about vaccines.

The danger of false equivalence remains for any issue on which a broad medical or scientific consensus exists based on the evidence and a handful of outliers attempt to discredit that information for various reasons, often motivated by personal financial gain. Avoiding false balance doesn’t mean journalists take off their skeptical hat in covering these issues, but they should only report these scientifically outlier positions if solid evidence supports it, not just because someone somewhere believes it.  

Incidence vs. prevalence

Many of the medical studies journalists cover are epidemiological, which are observational studies focusing on the health of populations. These studies tend to report on the incidence and prevalence of diseases and other conditions, so it’s important that journalists understand the difference between these two commonly confused terms in epidemiology.

In the plainest terms, “incidence” refers to new cases of a disease or injury or condition. “prevalence” refers to the total existing cases of a disease or injury or condition – whether newly occurring or ongoing from a previous diagnosis or occurrence. Although these terms can refer to any condition studied, such as gunshot wounds, short-term infectious disease or chronic conditions, this section will primarily focus on diseases for the sake of simplicity.

Whether a study, or a journalist, uses incidence or prevalence depends on what’s being communicated. For example, to have a sense of how quickly a disease is spreading through a population, incidence is more relevant because it describes new cases.

But to understand the burden of a disease, especially a chronic condition, in a population, prevalence is more relevant because it focuses on how many people are suffering, regardless of whether they were diagnosed yesterday or ten years ago.

These concepts involve more complexity, but first, here is a visual analogy to make sense of the difference: imagine a bathtub that has the faucet turned on and the drain open. The water pouring into the bathtub is the incidence – the new cases getting diagnosed. The water that is in the bathtub is the prevalence – how many currently have the condition. The water exiting the tub through the drain are the people leaving the prevalence either because they died from the condition or because they recovered from it.

Two more examples: If 500 people are diagnosed with diabetes each year, that refers to incidence, but if 15 million people are currently living with diabetes, that refers to prevalence. If 6 million people caught the flu in the first week of February, that’s incidence, but if only 4 million people are currently suffering from symptoms of the flu on February 7, that refers to the prevalence of influenza; the other 2 million recovered or died from the flu during that week.

Incidence is typically described in one of two ways: incidence proportion or incidence rate. The incidence proportion is also called cumulative incidence, attack rate, or risk of a condition — the probability of developing it. Incidence proportion is expressed as a ratio where the numerator (top number) is the total number of new cases of a condition during a specified time interval, and the denominator (bottom number) is the population of people who are at risk for the condition.

For example, the incidence proportion of HIV in a particular country might 25 people per 100,000 individuals per year. Similarly, the incidence of cervical cancer in the same country might be 10 per 50,000 women. Even though it’s the same population, the denominator must reflect the population that is at risk. Both males and females can get HIV, but only females can get cervical cancer, so the denominator can only include women in the second example. (The second example would probably actually be expressed as 5/100,000, but it’s important to know that the denominator still only contains women and that the HIV rate and the cervical cancer rates given here cannot be directly compared since the denominators refer to different populations within the same country.)

Even incidence proportion can be described different ways. For example, the overall incidence proportion, or attack rate, of a listeriosis outbreak refers to the total number of individuals getting newly diagnosed with the foodborne illness out of the total population. But if the source of the outbreak is determined to be cantaloupe, the food-specific attack rate refers to the number of new cases of illness among people who ate that food. (It can get even more specific if the denominator is limited to the people who ate the cantaloupe from the farm where the outbreak originated.)

The incidence rate is less familiar to journalists even though they will come across it in studies; it refers to the number of newly diagnosed cases in the population over a set amount of time. It’s often expressed in “person-years,” which incorporates time into the denominator. In writing about this type of incidence in layperson terms, one way express it is to do a quick division and use “cases per year” (or whatever the unit of time is, usually days or years). For example, if rate of norovirus in Pleasantville over a 10-year period is 25,000 cases per 1 million person-years, then that actually means the population is approximately 100,000 people (100,000 people times 10 years is 1 million person-years), and 2,500 people a year got sick. (If 25,000 cases occur over that time, the annual rate is estimated by dividing by 10.) The reason researchers might express a condition in person-years instead of annual rate is that the population might change over that time and person-years is more precise and accurate for researchers. Usually, for a journalist’s purposes, that level of precision is not necessary, and the estimate of 2,500 cases per year is sufficient.)

Prevalence can also be discussed in two different ways: point prevalence and period prevalence. Just as it sounds, point prevalence refers to the number of people with a certain condition at a precise moment in time, such as a day or “right now” throughout the U.S. The numerator is the number of current cases, and the denominator is the total current population. The estimated point prevalence of HIV in the U.S. is 1.2 million people. Meanwhile, period prevalence refers to the number of current cases over a period of time, such as over a year. The point prevalence of the flu in February may be 300,000 cases, but the period prevalence of the flu over the entire year might be 9 million (which includes the 300,000 cases in February). Whether this refers to the cases in a nation, a state, a county, a city, a school or some other group depends on the study and the needs of the journalist’s story.

For additional discussion of incidence versus prevalence, review this Paediatric Nursing primer or this explanation from the University of North Carolina School of Public Health (which includes an illustration of the bathtub analogy).

For more nitty gritty specifics, check out this lesson on the CDC website on Morbidity Frequency Measures.

One Health

One Health is a growing field within public health that embraces the connection between animals, humans and the environment and solve complex health problems such as emerging infectious diseases, food safety and antibiotic resistance.

The medical community observed that human and animal health were closely linked back in the late 1800s, but the concept of One Health has risen in prominence as the world’s population has exploded. By 2025, there are expected to be more than 8 billion people living on the planet, up from about 7.4 billion at the end of 2017.

Scientists estimate that more than 60 percent of all new emerging infections are a zoonosis, meaning they come from animals. Commonly known zoonoses include avian influenza, Ebola, rabies, Middle East Respiratory Syndrome and Lyme Disease. Worldwide, the number of infectious disease outbreaks has tripled. More than a dozen new infectious diseases have emerged over the past 25 years in the U.S. alone.

Outbreaks are associated with economic and agricultural turmoil. The 2014 outbreak of the Ebola virus, for example, cost Guinea, Liberia and Sierra Leone about $2.2 billion and a 2014 pathogenic avian influenza outbreak cost U.S. farmers about $3.3 billion.

Population growth is spurring antibiotic resistance. The increase in people is driving rising demand for animal protein, and an increase in animal production operations. In turn, antibiotics are being used on more animals, accelerating the rise of antibiotic-resistant bacteria, or “superbugs” in the environment. In the U.S., about 2 million people annually contract a superbug, and 23,000 die. By 2025, as many as 10 million people, could die annually as the result of a superbug infection, if humanity does nothing.

For all of these reasons, public health momentum surrounding One Health has grown. The U.S. Centers for Disease Control and Prevention created the first One Health office in 2009, to foster collaboration between international, federal, state and local governments, as well as the academic, health and private sectors.

As One Health is relatively new in the public health field, the definition of the term is imprecise. One Health has been defined as an initiative, a movement, a strategy, a framework, an agenda, an approach and a collaborative effort. In general, One Health involves the intersection of biology, comparative medicine, earth sciences, ecology, engineering, human medicine, social sciences, humanities and veterinary medicine. One Health programs link physicians, nurses, public health professionals, veterinarians, agricultural scientists, ecologists, social scientists, engineers, biologists and other professionals, to develop holistic solutions for keeping humans, animals and the environment healthy.

In the U.S., the CDC’s 10-person One Health office regularly coordinates discussions between the U.S. Department of Agriculture, the Department of Health and Human Services, the Interior Department, state health and agriculture departments, health systems, and health providers to discuss emerging infectious diseases that may impact communities.

Pandemic influenza

Influenza, a respiratory disease caused by a virus, is endemic to humanity.  The virus is always around, most often striking populations in the late fall or winter seasons. The flu virus attacks not only the respiratory system but also can cause headaches, muscle and joint pain and other complications. The overwhelming majority of the time, people who are infected with the flu recover within about ten days, which is why influenza is rarely viewed with great concern.

Still, the flu can seriously sicken and kill people, particularly older individuals or those with weak immune systems or who are in poor health. In the U.S., seasonal flu can kill between 12,000 to 56,000 people and can hospitalize between 140,000 to 710,000, according to the Centers for Disease Control and Prevention. Because so many people do get sick annually, public health officials urge the public to get an annual flu vaccine. The vaccine isn’t 100 percent effective, however, because flu viruses mutate.

The genetic code of a virus is always mutating to outwit a host’s immune system. Among the fastest mutating is influenza. Mutation occurs when the virus makes a “mistake” while reproducing itself. The mistakes are often minor and called antigenic drift. Sometimes several different influenza viruses come together and swap and rearrange their genetic material, creating a new hybrid virus. This change is called antigenic shift. Historically - several times a century, the flu virus shifts to become more lethal.   Because flu can spread so easily - the virus spreads through water droplets in by breath - public health officials most worry about a pandemic flu outbreak. Pandemic means the flu is spreading quickly throughout multiple countries.

Viruses aren’t strictly alive or dead. They are biological particles that lurk in the environment until they find a cell to latch onto and enter. The virus replicates inside the cell, until there are so many viruses, the cell explodes and the newly made viruses go on to enter other healthy cells. Influenza viruses originated in the gut of wild aquatic birds. Though bird viruses don’t spread easily among humans, they do spread between animal species, including domestic birds like chickens and turkeys and pigs. Because swine and humans have similar respiratory cell receptors, a mutating flu in a pig can jump more easily to and between people, as was the case in 2009, with the pandemic swine flu.

There are three types of influenza virus: A, B and C. The different letters stem from the different proteins that contain the virus’s genetic code. The seasonal flu comes from either Type A and B. Type C typically doesn’t cause disease in humans. Type A is found in humans, birds, pigs and other animals, while Type B is only found in humans. Research has shown that only Type A causes pandemics. Proteins that jut out of the virus’ outside covering identify type A viruses. There are eighteen different hemagglutinin or H proteins and eleven different neuraminidase or N proteins. The H proteins give the virus the ability to enter cell walls and the N protein governs the release of the newly minted viruses out of cells. Scientists classify flu viruses based on their H and N proteins. The 2009 flu virus, for example, was H1N1.

Between 1918 and 1919, the world was struck by the largest flu outbreak in modern history. Though it was called the “Spanish” flu, it may have actually started in the U.S. in an agricultural district in Kansas. It was called the Spanish flu because Spain was the first country to report on a widespread flu. Epidemiologists say the 1918 strain spread from an army base in Kansas, where recruits brought it to Europe and then the rest of the world. What made the flu particularly dangerous is that it sickened young people and pregnant women more often than the old and immune-compromised. The virus caused otherwise healthy people’s immune systems to overreact, damaging organs and killing them. The process is called a cytokine storm. Estimates range on how many people died worldwide, with the largest suggesting that about 100 million died. About 500,000-650,000 people are estimated to have died in the U.S.

Since 1500, there have been more than a dozen flu pandemics recorded, with at least five occurring in the past 140 years - in 1889, 1918, 1957, 1968, and 2009. 

None have been as deadly as the outbreak in 1918, but scientists fear another deadly outbreak is inevitable because they see signs that influenza is evolving more rapidly than ever. The explosion in worldwide bird and pig farming is giving flu viruses lots of opportunities to evolve and become dangerous to humans. Pharmaceutical companies can develop vaccines to prevent the spread of a dangerous flu, but it takes time - in the range of four to six months - to identify a flu and then manufacture a vaccine. By that time, a pandemic flu may have killed millions.

Quarantine and isolation

In the event of an outbreak of a contagious disease, health authorities may deploy several strategies to protect healthy people from getting sick, including implementing quarantines and isolation.

A quarantine involves restricting the moment of a person suspected of exposure to a communicable disease, even though the person isn’t yet showing any signs of illness, or doesn’t know if they might been exposed. The person is kept apart from the community until he or she can no longer transmit the disease to others. The time period of quarantine depends on the length of time a pathogen remains infectious.

Isolation involves separating someone who is already ill and removing them for anyone who isn’t sick, and keeping them apart until they are well. 

The history of quarantines goes back to the Middle Ages, when the plague was sweeping through Europe. Venice, a major port, tried to stop the disease from entering its city by requiring ships suspected of harboring plague, to wait offshore for 40 days before people or goods could come ashore. The city also built a hospital off its coast, where sailors who came off ships with the plague were sent. The forty-day waiting period was called “quarantinario,” for the Italian word for forty. Hence the word “quarantine.”

Quarantines can be important when there is no vaccine or drug to treat a rapidly spreading disease. They are, however, very controversial because they involve separating healthy people from the community, and they raise civil liberties questions. Forced quarantines and isolation can cause societal panic, as people worry about getting food, losing work, or becoming isolated from others.

Historically, quarantines have been used to target vulnerable populations in society, such as ethnic groups and the poor. In 2014, during the outbreak of Ebola (a viral hemorrhagic fever with no cure or vaccine), Liberia tried to impose a quarantine for 21 days (the incubation period for Ebola), causing people to flee into the jungle. The quarantine also sparked intense protests, leading the government to end the quarantine in ten days.

In the U.S., the federal government, through the Centers for Disease Control and Prevention, has the legal authority to quarantine and isolate a person, for up to 72 hours, at a U.S. airport, port or the border if the person is known to be infected or possibly infected with one of nine quarantinable diseases. The nine include: cholera, diphtheria, infectious tuberculosis, smallpox, yellow fever, viral hemorrhagic fevers, severe acute respiratory syndrome, new types of flu that could cause a pandemic, or a disease that has been designated by order of the President.

Health providers living in the U.S. who traveled to West Africa to care for Ebola patients in 2014, were asked to voluntarily remain at home and monitor themselves for any signs of illness during the 21-day incubation period. Ebola isn’t contagious until a person shows symptoms of illness. But some states when farther than the federal government during the outbreak.

Under emergency preparedness powers, every state, the District of Columbia and most territories have laws authorizing the use of quarantines and isolation, usually through the state’s health authority. Some states, including New York and New Jersey imposed forced quarantines upon some returning health care workers including Laura Skrip, a public health graduate student who had been in Liberia providing computer technology support during the outbreak. She was forced to stay in her apartment in isolation, enforced by a police officer patrolling her building.

The CDC has recently updated its community guidelines on quarantine in the event of a flu pandemic, which could provide guidance in future outbreaks of other diseases.


R0 (pronounced R-naught) is a number epidemiologists use to determine the infectiousness of a disease and a community’s susceptibility to an epidemic. The “R” stands for “reproductive number” and is a kind an epidemiological threshold. If the “R” number of a bacteria, virus, fungi or parasite, is greater than 1, the pathogen has a greater chance of spreading through a population and causing an epidemic. If it is less than one, then it is likely the disease will die out. If you want to geek out on the mathematical forumal, check out the Mathematical and Statistical Estimation Approaches in Epidemiology or this detailed description.

Understanding the susceptibility of a population is important for helping public health officials determine strategies for controlling the spread of an infectious disease, such as vaccinating the population or quarantining sick individuals if no vaccine is available.

The “R” number for a disease is a range and changes with conditions within the community at the time. Many factors impact the “R” number including the period of time for which a disease is contagious (the longer a person is contagious, the more likely the disease is to spread), the number of people that a sick person comes in contact with (a sick person who stays home may spread the disease more slowly), how the disease is transmitted between people (diseases that spread through the air, like measles can travel quickly, while those that are sexually transmitted spread more slowly), the immunity level of the population (whether people have been vaccinated for the disease or survived a version of the disease in the past) and whether there is a strong health and legal system within the community (hospitals to treat people and law enforcement to impose quarantines can reduce spread of a disease).

Ebola is a good example of how conditions within a community impact the “R” number. Ebola is spread between people by an infected person’s blood or bodily fluids. The World Health Organization says the average mortality rate from Ebola, is about 50 percent, but can range between 20 percent to 90 percent. The R number for Ebola is estimated around 1.5 to 2.5.

During the 2014 Ebola outbreak in West Africa, the R number was 1.51 in Guinea, 2.53 in Sierra Leone and 1.59 in Liberia. In Nigeria, the number was below 1, because as soon as one Ebola patient was identified, the country implemented a tracing program to isolate exposed individuals. There was no outbreak in the country.

Infectious diseases with R numbers that average above 1 include Hepatitis C (R2), Ebola (R2), HIV/AIDS (R4), Severe Acute Respiratory Syndrome (SARS) (R4), mumps (R10) and measles (R12-18) Healthline has a description and graphic here.

The R-naught number emerged into culture in the 2011 movie “Contagion” in which a virus causes a deadly epidemic in the U.S. Actress Kate Winslet plays a Centers for Disease Control and Prevention official who writes the R-naught formula onto a white board to try to determine how quickly the disease is spreading. Here’s an interesting Q & A from Wired magazine on the science behind the movie, which includes a conversation about the R-naught figure.


Vaccines are agents (usually dead or weakened microorganisms, or a genetic piece of the organism) that elicit a specific immune response protecting individuals from the pathogen should he or she be exposed to it at a later date. Vaccines stimulate protection without triggering the disease. They are considered the most important and powerful tools for preventing the spread of infectious diseases. Vaccines are estimated to save about 3 million lives worldwide annually.

The term vaccine stems from the work of Edward Jenner, an English physician, who noted in the mid-18th century that people exposed to cowpox, a mild version of smallpox, were then immune to smallpox. Smallpox, a virus that causes fever and severe and debilitating skin blisters, was killing about 400,000 Europeans annually during the 18th century. The Latin translation of cowpox was “variolae vaccinae.”

In 1796, Jenner experimented with taking a scab from a milkmaid with cowpox and inserted it into a cut on an 8-year-old boy. The boy became immune to smallpox, proving a person could be protected from smallpox without being directly exposed to it. Jenner’s method was recognized as the first scientific attempt to control an infectious disease. Breakthroughs in science after Jenner’s experiment led to the development of vaccines for rabies (1885), plague (1897), typhoid (1899), cholera (1911), diphtheria (1914), tuberculosis (1921), whooping cough (1940), polio (1955), measles (1963), mumps (1967) and rubella (1969).

The impact on public health is obvious. A little more than a century ago, the U.S. infant mortality rate was 20 percent and childhood mortality before the age of five was 20 percent.  Many Americans knew the distinct sound of a child with whooping cough gasping for breath, and saw children with congenital defects caused by rubella or paralyzed by polio. Public health campaigns in the 1960s to require all children to be vaccinated succeeded spectacularly. Only a handful of children get diphtheria and rubella annually and polio has been eradicated in the U.S.  Among the top cases of death in the U.S. in 1900 were tuberculosis and diphtheria. Cancer and strokes have replaced them as of 2014.

The World Health Organization, the public health arm of the United Nations, in the 1960s made a push to eliminate most childhood diseases by vaccinating as much of the world as possible. One of the organization’s most successful achievements was the elimination of smallpox. The last naturally occurring case of smallpox occurred in Somalia in 1977.

Each type of vaccine is different in terms of composition and formula. The differences reflect not only the pathogen from which the vaccine is derived but also how the vaccine is used and how the body responds. Some vaccines are living and weakened versions of the pathogen. These are called “attenuated” vaccines. Measles, mumps rubella and some polio vaccines are an example of attenuated vaccines.

Other vaccines are inactive and consist of either dead bacteria or viruses – often referred to as whole cell vaccines – or are pieces of the microorganism. They are often given in multiple doses. Inactive vaccines include some flu vaccines, rabies, whooping cough, tetanus, hepatitis A and B.

Some vaccines are composed of a toxin extracted from the microorganism. Vaccines for diphtheria are an example. Researchers are also working on developing vaccines using just portions of a pathogen’s DNA to stimulate immunity.

All 50 states and the District of Columbia require children to be vaccinated – typically for diphtheria, measles, rubella and polio – before they can attend public school, but some states allow exemptions for medical, religious or philosophical reasons.

Vaccines have worked so well that many people take them for granted. In the past two decades, a small but growing number of parents have been refusing vaccinations for their children due to fears that vaccines cause autism. The fears stem from a now discredited study that a British scientist said demonstrated an association between autism and vaccination. Countless studies have shown the safety of vaccines. Nevertheless, childhood vaccination rates have fallen. Measles outbreaks have flared, most recently in Minnesota, where unvaccinated children in a Somali community contracted the disease.

Some celebrities and prominent leaders have supported parental fears. President Trump invigorated the anti-vaccine movement when he suggested during a 2015 Republican presidential debate that there is a link between vaccines and autism. He said in January 2017 that he is considering creating a commission to look into vaccine safety. As of August 2017, no such commission had been created.