The Manual on Viruses

What Is the Flu?
Influenza is a viral respiratory infection that causes symptoms similar to, but
more severe than, the common cold. Flu symptoms can include sudden onset
fever, cough, runny or stuffy nose and severe malaise (feeling unwell). The
flu can cause serious complications, including pneumonia, bronchitis and
sinus and ear infections.
Influenza viruses, which are part of the Orthomyxoviridae family of viruses,
cause the flu. There are four types of influenza viruses: A, B, C and D. Human
influenza A and B viruses cause seasonal epidemics of disease (known as
the flu season) almost every winter in the United States. Influenza A viruses
are the only influenza viruses known to cause flu pandemics, i.e., global
epidemics of flu disease. A pandemic can occur when a new and very different
influenza A virus emerges that both infects people and has the ability
to spread efficiently between people. Influenza type C infections generally
cause mild illness and are not thought to cause human flu epidemics. Influenza
D viruses primarily affect cattle and are not known to infect or cause
illness in people.
Influenza A virus, which also infects including birds, swine, horses, and other
animals, is further divided into subtypes based on two antigens (proteins)
on the virus’s surface: hemagglutinin (H), of which there 18 subtypes, and
neuraminidase (N), of which there 11 subtypes.
The specific virus is recognized by these antigens. For example, H1N1 refers
to influenza A virus with hemagglutinin subtype 1 and neuraminidase
subtype 1, and H3N2 refers to influenza A virus with hemagglutinin subtype
3 and neuraminidase subtype 2. Influenza B viruses circulate widely only
among humans.
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