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.
Human influenza is a member of the Orthomyxoviridae family. There are three known genera. Each has an alphabetical code: Influenza A, B, and C. (Influenza D was identified in 2016, but so far only infects cattle.)
Type C viruses cause mild symptoms and are not considered dangerous.
Type B viruses can cause serious illness and can lower one's resistance so bacterial infections can cause complications such as pneumonia. Influenza B viruses do not mutate or change into new forms. They are identified as different lineages and strains. The current circulating influenza B viruses belong to one of two lineages: B/Yamagata and B/Victoria.
Type A viruses do mutate, which makes them the most dangerous. Influenza A viruses are divided into subtypes. Based on two proteins on the surface of the virus: hemagglutinin (H) and neuraminidase (N). These two surface proteins combine to make 18 different hemagglutinin subtypes (H1 through H18) and 11 different neuraminidase subtypes (N1 through N11).
Infection with one type gives no protection against another; and the virus may continually change its character, making it difficult to prepare an effective vaccine. Influenza A H1N1 and H3N2 are the most common types of seasonal flu viruses. The flu vaccine contains these types of influenza viruses.
The A-type disease also affects animals and birds. Some influenza-A viruses may be classed as zoonoses, as it appears that new strains affecting man may have originated in animals. Aquatic birds, gulls and ducks are the main reservoir for the influenza virus. Although the birds are usually not made ill by the viruses they carry, the birds can infect humans directly or, more often, infect domestic fowl and animals such as pigs, from which humans acquire the infection. It is from those sources that another super-flu virus can emerge, like the devastating human flu epidemic following World War I, during which more soldiers (over 40 million) died of influenza than on the battlefield. This virus has been called a "super-virus", which did all the things an influenza A virus does, only better: it infected people faster, it spread faster and it caused more deaths from pneumonia and other complications. 2 billion people suffered from the disease but recovered. This pandemic caused more death and destruction in a six month period than any other before or since.
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.