Flu (influenza) is caused by one of several strains of viruses that infect the nose, throat and lungs. Flu-like symptoms make life miserable for a week or two for many people, but they can be deadly for some. In a typical year, the seasonal flu affects between 5 and 20 percent of the population in the United States, and about 36,000 people die from complications of the flu. Flu season can begin as early as October and peak anywhere from late December to early April.
Your best defense against the flu is to get vaccinated as soon as possible when the flu vaccine becomes available in your community. During the flu season, you can get vaccinated with a flu shot, given by a needle. This form of the vaccine contains killed virus and is approved for all people older than six months.
Influenza viruses are divided into three types designated as A, B and C:
- Influenza types A and B cause epidemics of respiratory illness that happen almost every winter. They often lead to increased rates of hospitalization and death. Public health efforts to control the impact of influenza focus on types A and B. One of the reasons the flu remains a problem is because the viruses actually change their structure regularly. This means that people are exposed to new types of the virus each year.
- Influenza type C usually causes either a very mild respiratory illness or no symptoms at all. It does not cause epidemics and does not have the severe public health impact that influenza types A and B do.
Influenza viruses continually change (mutate), which helps the virus to evade the immune system of both children and adults. The process works like this:
- A person infected with an influenza virus develops antibodies against that virus.
- The virus changes.
- The "older" antibodies no longer recognize the "newer" virus when the next flu season comes around.
- The person becomes infected again.
The older antibodies can give some protection against getting the flu again. Vaccines given each year to protect against the flu contain the influenza virus strain from each type that is expected to cause the flu that year.