The ABCs of the flu.

• Position: Influenza

Influenza, usually called the flu, is a viral infection of the respiratory tract. The flu can move from the nose or mouth to the rest of the respiratory system. It usually is spread by touching an infected person or contaminated surface or by inhaling droplets coughed or sneezed into the air. If a bacterial infection develops in addition to the viral one and travels from the upper respiratory tract to the lungs, it can cause more serious disorders, such as acute bronchitis or pneumonia. Influenza kills about 36,000 people a year in the U.S.

According to the American Medical Association Family Medical Guide, there are three main types of influenza virus: A, B, and C. If you have had the flu caused by a type C virus (a relatively mild strain with symptoms similar to those of a cold), you are immune to it for life. If infected with a particular strain of a type A or B influenza virus, you have immunity to that strain only. Although A and B influenza viruses can produce new strains that can overcome a person's immunity, the type B virus seldom alters itself sufficiently to do so. The type A virus, however, changes constantly, and the mutations are significant enough to make it look like a new virus to the immune system. For this reason, type A viruses cause most flu epidemics and severe outbreaks. These strains usually are named after their place of origin (such as the Hong Kong flu).

Influenza usually occurs in small outbreaks, often in the winter. Every few years, in unpredictable intervals, it spreads in epidemics. Two or three epidemics caused by different strains of the virus can occur at the same time. Epidemics die out when everyone who has been infected by a particular strain of a flu virus becomes immune to that...