Most cases occur during epidemics, which reach peak intensity during the winter months nearly every year. 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 influenza during flu season. Type C usually produces only a mild infection and often causes no noticeable symptoms.
Most people get multiple influenza infections during their lives.
The immune system's attack mechanism is slow to recognize the invader, and by the time it acts, millions of the body's cells already have been infected with the virus. Most of the symptoms of the illness are caused by the chemicals the immune system uses to mount its mass attack. The more severe and widespread pandemics occur when the changes in the virus are substantial rather than subtle ones. Symptoms Influenza can produce a variety of symptoms, and they may be mild or severe depending upon the type of virus and your age and overall health. Although it is a respiratory virus, influenza can affect other body systems, making you feel sick all over.