An acute infectious disease, characterized by a sudden onset and causing fever, sore throat, muscle pains, cough, lassitude, and headache. Influenza usually occurs in epidemics and pandemics. In most cases the disease lasts 2-4 days and is characterized by headache, generalized aching and prostration symptoms. Even after an attack of only average severity there tends to be a period of weakness and depression.
The 1918 super-flu virus ("Spanish flu") probably first appeared in March at Army camps in Kansas. By July it was epidemic at several US military bases and the communities around them. Troop ships packed with soldiers bound for Europe proved ideal environments for the virus to spread from man to man. It spread next to France and Britain, and from there, around the world. Late in the summer, the virus suddenly seemed to become even more dangerous. It had been concentrated at military installations and in northeastern US cities. In September and October, it spread across the country; it may have mutated in some way and was more deadly than any influenza before or since. People who felt well at breakfast were ill by noon, and dead by evening. It seemed to target healthy younger people. It peaked in October, and then began mysteriously to subside, but by year's end, 2 Americans in every 5 had caught the flu, and before it was over in April 1919 some 650,000 people had died. The death rate, 5 out of every 1,000 Americans, was unprecedented, and for some reason, mortality was twice that high among healthy younger people. American forces in Europe suffered about 115,000 casualties during World War I. Some 43,000 resulted not from enemy fire but from the flu. In the United States and other countries where it had first appeared, the super-flu virtually disappeared, as suddenly as it had appeared, during the spring of 1919. It continued to spread elsewhere, until every nation on Earth was affected. In India, it is believed that 12.5 million died. Some small communities, in remote areas such as Alaska and Polynesia, lost four-fifths of their populations.
In some years, influenza presents the largest viral threat on the planet.