Health A-Z

Medical Content Created by the Faculty of the
Harvard Medical School

Flu (Influenza)

What Is It?

Flu, known medically as influenza, is a respiratory infection caused by the influenza virus. The infection typically is spread by air or by direct contact from one person to another. Most cases occur during epidemics, which peak during the winter months nearly every year. Influenza virus is very contagious. A particularly widespread and severe epidemic is called a pandemic.

Compared with other viruses, influenza can strike remarkably large numbers of people in a relatively short time. Each year, about 25 million people seek medical care for the symptoms of flu during flu season. In the United States, the Asian flu of 1957-1958 caused 70,000 deaths, and the Hong Kong flu of 1968-1969 killed 34,000 people. In the worst recorded pandemic of influenza, the 1918-1919 Spanish flu, 20 to 40 million people throughout the world died in less than one year.

The most common types of influenza virus are A and B. Influenza A is the one usually responsible for the annual epidemics. Most people get multiple flu infections during their lives. With many other types of infections — for example, mumps — having the disease once protects against a second infection because the body's immune system "remembers" the returning virus, attacks it immediately and rapidly eliminates it. With influenza, the virus usually has mutated (changed) somewhat since the first infection, and the change is enough to fool your immune system. Instead of attacking the virus rapidly, as it would a virus that it had seen before, the immune system responds slowly. By the time the immune response is in full gear, millions of the body's cells already have been infected with the virus.