Influenza is an infectious disease caused by the influenza virus. The term influenza is used to describe influenza and other similar illnesses. The general symptoms of influenza, also known as the flu or grippe, are chills, headache, fever, weakness and aching of the joints. The incubation period is short--between one and three days--and the first symptom is a fever that may reach 103° F. Cough and gastrointestinal discomfort may also accompany the disease. Usually, the viral infection runs its course in about a week. However, the virus weakens the immune system, making the human body subject to secondary infections such as bacterial pneumonia. Fatalities associated with influenza usually result from such secondary complications. The presence of viral pneumonia was probably the cause of many deaths during the great influenza epidemics of the past.

Historically, influenza has been known for centuries and may have been what Hippocrates of Cos...