Influenza is a highly contagious viral infection. There are three main antigen types of influenza: types A, B, and C. Influenza type A causes moderate to severe illness and affects all age groups. Type A can infect both humans and animals. The notion that influenza type B generally causes milder disease than type A has been recently challenged and is more fatal in children. Type B mainly infects humans. Influenza type C only causes mild respiratory illness. Influenza activity peaks from December to March in the Northern Hemisphere.
Influenza is transmitted from person to person mainly when an infected person coughs or sneezes. Transmission can also occur from direct contact with respiratory secretions, such as when touching surfaces contaminated with influenza virus and then touching the eyes, nose, or mouth, or from indirect contact with droplets from coughs or sneezes. Transmission can occur from the day before symptoms begin until about 5 days after symptoms begin in adults, or up to 10 days or more after symptoms begin in children.
About 50% of infected persons will develop "classic" influenza disease. This is characterized by the abrupt onset of fever, muscle pain, sore throat, nonproductive cough, runny nose, and headache. These symptoms generally last from 2 to 3 days. However, some people may experience a loss of strength or energy for several weeks. In addition, some people may develop complications, such as pneumonia.
Source: Centers for Disease Control and Prevention