Influenza is an acute infection caused by any of three types of viruses (A, B, or C). Type A strains are associated with the most severe disease. Flu is an acute infection of the airway tract in the nose and throat that can sometimes spread down into the lungs. Flu in adults is a frequent cause of acute respiratory illness. Flu, however, affects people of all ages. Children are among the groups most at risk for developing flu and its complications and are more likely to spread the infection to others. Flu season begins every year mainly in late fall and early winter and usually spreads widely. The peak season for the flu in the northern hemisphere is from November through March.
The flu is caused by one of three types of influenza viruses. Types A and B are responsible for the yearly flu epidemics, and type C causes sporadic illness. Type A is further divided into different subtypes based on the chemical structure of the virus. Influenza is highly contagious. The virus is spread when someone either inhales infected droplets in the air (coughed up or sneezed by an infected person) or when someone comes in direct contact with an infected person's secretions (for example, kissing, sharing of handkerchiefs and other items, and through use of objects such as spoons and forks).
