An infection happens when a foreign organism enters a person's body and causes harm.






Fast facts on infection

  • Infection is the effect of a foreign organism in the body.
  • They are classified by the type of organism causing the infection.
  • Infections can range from mild inflammation in one person to an epidemic.

Infection bacteriaShare on Pinterest
Bacteria are one type of infectious agent.




Viral infections

Viral infectionShare on Pinterest
The common cold is a viral infection.






Here are some examples of viral infections:

Other viral conditions include:

Antiviral medications help in some cases. They can either prevent the virus from reproducing or boost the host's immune system.



Bacterial infections

Bacteria are single-celled microorganisms known as prokaryotes.

There are estimated to be at least one nonillion bacteria on Earth. A nonillion is a one followed by 30 zeros. Much of Earth's biomass is made up of bacteria.

Bacteria take three main shapes:

  • Rod-shaped: These are called bacilli.


There are trillions of strains of bacteria, and few of these cause diseases in humans. Some of them live inside the human body without causing harm, for example in the gut or airways. Some "good" bacteria attack "bad" bacteria and prevent them from causing sickness.

However, some bacterial diseases are deadly.

These include:

Some examples of bacterial infections are:


Fungal infection

Infection fungusShare on Pinterest
Fungi reproduce by spreading spores.



There are approximately 51 million species of fungus.

Many fungal infections will appear in the upper layers of the skin, and some progress to the deeper layers. Inhaled fungal spores can lead to systemic fungal infections, such as thrush, or candidiasis. Systemic diseases affect the whole body.

The body usually has a population of "good" bacteria that help to maintain the balance of microorganisms in the intestines, mouth, vagina, and other parts of the body.

If enough "good" bacteria are destroyed, for example, by overusing antibiotics, fungi can grow and cause health problems for the host.

Those with a higher risk of developing a fungal infection include people who:

  • use strong antibiotics for a long time
  • have a weakened immune system, due, for example, to HIV or AIDS, diabetes, chemotherapy treatment, and those who have undergone a transplant, as they take medications to prevent their body from rejecting the new organ

Examples of fungal infections are:

  • valley fever, or coccidioidomycosis
  • athlete's foot
  • ringworm
  • some eye infections

A rash can be an indicator of a fungal infection of the skin.

Prion disease


Prions do not replicate or feed on the host but trigger abnormal behavior in the body's cells and proteins.


Prions cause degenerative brain diseases, such as:

Researchers have linked some cases of Alzheimer's disease to prion infection.

Other infections

While the forms of infection mentioned above are the main types, there are others that can have an effect on the body.

A single-celled organism with a nucleus can cause a protozoan infection. Protozoa commonly show features similar to animals, such as mobility, and can survive outside of the human body. They are most commonly transferred by contact with feces.

When they enter the human body, protozoa can also cause infection. Amebic dysentery is an example of a protozoan infection.

Helminths are larger, multicellular organisms that tend to be visible to the naked eye when full-grown. This type of parasite includes flatworms and roundworms. These are also able to infect the human body.

Finally, ectoparasites such as mites, ticks, lice, and fleas can cause infection by attaching or burrowing into the skin.


The cause of an infection is said to be whichever type of organism has invaded the body.

A particular virus, for example, will be the cause of a viral infection.

The effects of an infection, such as swelling or a runny nose, occur as a result of the immune system fighting the invading organism. A wound filling with pus, for example, occurs when white blood cells rush to the site of an injury to combat foreign bacteria.

The symptoms of an infection depend on the organism responsible as well as the site of the infection.

Viruses target specific cells, such as those in the genitals or upper respiratory tract. The rabies virus, for example, targets the nervous system. Some viruses target skin cells, causing warts. Others target a wider range of cells, leading to various symptoms. A flu virus can cause a runny nose, muscle aches, and an upset stomach.

A person with a bacterial infection will often experience redness and heat, swelling, fever, pain at the site of infection, and swollen lymph glands. A bacterial infection is less likely to affect a wide area of the body than a viral one.


Common symptoms of prion diseases include brain damage, memory loss, and cognitive difficulties. They can also trigger the buildup of plaque in the brain, causing it to waste away.

There is no single way to prevent all infectious diseases, but the following tips can reduce the risk of transmission:

  • Only take antibiotics when prescribed, and be sure to complete any recommended course even if symptoms improve earlier than anticipated.
  • Disinfect rooms where there may be high concentrations of bacteria, such as the kitchen and bathroom.
  • Practice safe sex by receiving regular STD checks, using condoms, or abstaining altogether.
  • Avoid sharing personal items such a toothbrushes, combs, razorblades, drinking glasses, and kitchen utensils.

A healthy, active lifestyle can help keep the immune system strong and able to defend the body against different kinds of infection.