Influenza is a viral infection of the lungs characterized by fever, cough, and severe muscle aches. In the elderly and infirm, it is a major cause of disability and death (often as a result of secondary infection of the lungs by bacteria). Even in the young and healthy, influenza produces a prostrating disease of a few days duration and one not soon forgotten.

Influenza is not

* a case of low fever and sniffles that keeps you home in bed for a day
*a gastrointestinal upset (“stomach flu”)

Influenza was responsible for the most devastating plague in human history — the “Spanish” flu that swept around the world in 1918 killing 675,000 people in the U.S. and an estimated 20-50 million people worldwide. (A disease that attacks a large fraction of the population in every region of the world is called a pandemic.) (It is uncertain where the flu first appeared, but it certainly wasn’t in Spain.)

No one at the time even knew what disease agent was causing the pandemic. Not until 1930 (in pigs) and 1933 (in humans) was it established that influenza is caused by a virus. This electron micrograph (courtesy of Dr. K. G. Murti) shows several influenza virus particles (at a magnification of about 284,000x). The surface projections are molecules of hemagglutinin and neuraminidase (see below).

There are three types of influenza:

*Influenza C
Common but seldom causes disease symptoms

*Influenza B
Often causes sporadic outbreaks of illness, especially in residential communities like nursing homes.

*Influenza A

Responsible for regular outbreaks, including the one of 1918. Influenza A viruses also infect domestic animals (pigs, horses, chickens, ducks) and some wild birds

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