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.
Influenza virus is dynamic. It spreads across different animal species, not just infecting humans. There are three main types: Influenza A, B, and C. Influenza A is subdivided into H and N subtypes, creating 144 possible combinations. The evolutionary path keeps going and new variants emerge all the time.
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.
Flu epidemics appear to occur at approximate ten year intervals; for example 1947, 1957, 1968, 1997 (Hong Kong (bird) flu). Earlier series include 1889, 1918, and the mid 1930s.
The influenza season is usually from November to March in the northern hemispheres, and is reversed in the south (May to October). In the tropics, the virus can be isolated year around and epidemics of disease can occur at variable times of the year, including the summer months. Annual attack rates average 10% to 20%, but may be higher during severe epidemics.
Malaise following influenza can persist for several weeks. Morbidity and mortality, associated with influenza, are usually more common in the older population and in individuals with significant concurrent medical problems. These latter groups have been traditionally targeted for the immunization programmes. Vaccines provide a high degree of protection, but are not 100 percent effective, especially for older people and those of any age whose immune systems have been damaged.
Every year, 36,000 Americans die of the flu and one million die worldwide. In the United States, it has been estimated that influenza causes millions of lost days from work, and is responsible for 5 to 10% of US mortality.
Travelling and travellers may represent an important combination of exposure to the virus and risk for influenza.
Novel influenza viruses appear all the time. Most are not capable of causing a pandemic. They may cause illness in a few people, but they do not have the 1918 virus's ability to go on and infect large numbers. The 1957 Asian flu and the 1968 Hong Kong flu were caused by novel viruses, and reached epidemic levels in several countries, but never reached pandemic proportions. A novel virus of what at the time was called "swine flu" appeared in 1976 at Fort Dix, NJ, USA. It turned out not to be a rapidly spreading kind, but it caused a brief pandemic panic.