Influenza, also known as flu, grip, and grippe, is a disease of humans, pigs, horses, and several other mammals, as well as of a number of species of domesticated and wild birds. Among humans it is a very contagious respiratory disease characterized by sudden onset and symptoms of sore throat, cough, often a runny nose, and (belying the apparent restriction of the infection to the respiratory tract) fever, chills, headache, weakness, generalized pain in muscles and joints, and prostration. It is difficult to differentiate between single cases of influenza and of feverish colds, but when there is a sudden outbreak of symptoms among a number of people, the correct diagnosis is almost always influenza.

There is at present no specific cure that is effective against this viral disease. In mild cases the acute symptoms disappear in 7 to 10 days, although general physical and mental depression may occasionally persist. Influenzal pneumonia is rare, but often fatal. Bronchitis, sinusitis, and bacterial pneumonia are among the more common complications, and the last can be fatal, but seldom is if properly treated. Influenza is generally benign, and even in pandemic years, the mortality rate is usually low – 1 percent or less–the disease being a real threat to life for only the very young, the immunosuppressed, and the elderly. However, this infection is so contagious that in most years multitudes contract it, and thus the number of deaths in absolute terms is usually quite high. Influenza, combined with pneumonia, is one of the 10 leading causes of death in the United States in the 1980s.