Printer-friendly version

Pathology and causes of Flu (Influenza)

The flu is a contagious respiratory illness caused by influenza viruses. It can cause mild to severe illness, and can lead to death in severe cases. For people with asthma, guarding against the flu is important every year because your lung function can be very compromised if you catch the flu, making your asthma symptoms even worse.

Most healthy people recover from the flu without complications, some people, such as older people, young children, and people with certain health conditions like asthma, are at high risk for serious complications from the flu. Some of the complications caused by the flu include bacterial pneumonia, dehydration, and worsening of existing chronic conditions, such as congestive heart failure, asthma, or diabetes. Children and adults could also develop sinus problems or ear infections.

People at high risk for complications include people 65 years or older, people with chronic medical conditions, pregnant women and young children. If you are at high risk from complications of the flu, you should consult your health-care provider to learn how to prevent the flu. If you develop flu-like symptoms, seek professional medical help.

H1N1 ("Swine") Flu

H1N1 ("Swine") flu is a new strain and it’s different than the seasonal flu. But, like the seasonal flu, people with asthma are at a greater risk of getting this severe strain, so click on the link below to learn about what you need to know to prevent and prepare. (Note: For the 2010-2011 flu season, the flu vaccine does include protection against the 2009 H1N1 pandemic virus and 2 other flu viruses.) For more H1N1 flu information visit

Seasonal Flu

Every year the seasonal flu is responsible for causing complications for people with asthma. Getting seasonal flu shot each year is a very effective way to reduce your chances of dealing with complications due to flu symptoms. (Note: For the 2010-2011 flu season, the flu vaccine does include protection against the 2009 H1N1 pandemic virus and 2 other flu viruses.) For more H1N1 flu information visit

"Swine" Flu (the H1N1 Strain)

Information from the U.S. Centers for Disease Control and Prevention (CDC) suggests that many of the signs and symptoms of swine flu are similar to those experienced by people with seasonal allergies. If your runny nose, itchy and watery eyes, sore throat, and other common allergy symptoms are combined with an unusually high fever (100 degrees or higher), chills, severe headache, or significant aches and pains, you could have some type influenza, including swine flu.

Swine flu symptoms are similar to typical flu symptoms and to common allergy effects as well. As an asthma and allergy patient, you must be particularly vigilant to avoid contact with people who might have any strain of influenza, including swine flu.

You can take several simple precautions to protect yourself and the members of your family from contracting influenza. The CDC recommends taking the following steps:

  • Avoid touching your eyes, nose or mouth
  • Try to avoid close contact with sick people

If you think your allergy symptoms might be indicative of something more severe, including any strain of influenza, you should visit your primary care physician for a diagnosis or treatment recommendation. Further, if you become ill and experience any of the following warning signs, seek emergency medical care:

Emergency warning signs that need urgent medical attention for children include:

  • Fast breathing or trouble breathing
  • Bluish skin color
  • Not drinking enough fluids
  • Not waking up or not interacting
  • Fever with a rash

Emergency warning signs that need urgent medical attention for adults include:

  • Difficulty breathing or shortness of breath
  • Pain or pressure in the chest or abdomen
  • Sudden dizziness
  • Confusion
  • Severe or persistent vomiting

Causative agent

Influenza (flu) is an acute illness of the respiratory tract caused by influenza viruses. It is usually more common in periods from January to March and from July to August in Hong Kong. There are three known types of influenza viruses, namely A, B and C. Influenza A viruses can further be subtyped on the basis of two surface antigens: haemagglutinin (H) and neuraminidase (N); influenza A (H1N1) 2009 virus (i.e. human swine influenza virus) is one of them. In the spring of 2009, influenza A (H1N1) 2009 virus emerged to cause illness in human and resulted in a pandemic in mid 2009. In August 2010, the World Health Organization (WHO) announced that the world had moved into the post-pandemic period and the virus was expected to continue to circulate in the community as a seasonal influenza strain for some years to come. Influenza A (H1N1) 2009 virus has now become one of the seasonal influenza strains in Hong Kong.

New subtype variants appear from time to time and at irregular intervals. Antigenic drifts (minor changes) of influenza viruses lead to the emergence of new viral strains every year. These minor changes cause seasonal influenza and explain why reformulation of the influenza vaccine is required every year.

The occurrence of influenza pandemic results from the emergence of a pandemic strain, which appears when an antigenic shift (major change) occurs in the surface antigens of the influenza viruses. People will have very little or no immunity to the virus, causing the virus to spread more rapidly and extensively than a seasonal influenza virus would.

Influenza pandemic occurs roughly every 10 – 50 years and may strike any time of the year. It is usually associated with a large number of cases, higher severity of conditions, a higher death toll and consequently greater social and economic disruption.

Common Flu Symptoms

The flu usually starts suddenly and may include these symptoms:

  • Fever (often very high 101 or above)
  • Headache
  • Extreme tiredness, chills
  • Constant cough
  • Sore throat
  • Runny or stuffy nose
  • Body aches in bones and/or musles

Diarrhea and vomiting also can occur but are more common in children. These symptoms are referred to as "flu-like symptoms." A lot of different illnesses, including the common cold, allergy symptoms and asthma symptoms can sometimes be similar and confusing. Always consult with your doctor to make a proper diagnosis.


Healthy individuals can usually recover from seasonal influenza in 2 to 7 days. Symptoms may include fever, cough, sore throat, runny nose, muscle pain, fatigue and headache; some may also have vomiting and diarrhea. 

Cough is often severe and prolonged but fever and other symptoms generally subside in 5 to 7 days. However, influenza can be a serious medical condition to the elderly or those with weaker immunity, and may be complicated by bronchitis, chest infection or even death.

How Does the Flu Spread?

The flu spreads in respiratory spraying from coughing and sneezing. It usually spreads from person to person, though occasionally someone could become infected by touching something with virus on it and touching their mouth or nose. Adults may be able to infect others 1 day before getting symptoms and up to 7 days after getting sick. So it’s possible to give the flu before you know you’re sick, as well as while you are sick.

Incubation period

Usually around 1 to 4 days.

Infectious period

Infected persons may pass the viruses to other people 1 day before and up to 5 to 7 days after they develop symptoms. The period may be even longer in young children or people with a seriously compromised immune system.