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Cause of Influenza
Also called flu or grippe an acute viral infection of the upper or lower respiratory tract that is marked by fever, chills, and a generalized feeling of weakness and pain in the muscles, together with varying degrees of soreness in the head and abdomen.
Influenza is caused by any of several strains of orthomyxoviruses, categorized as types A, B, and C. The three major types generally produce similar symptoms but are completely unrelated antigenically, so that infection with one type confers no immunity against the others. The A viruses cause the great influenza epidemics, and the B viruses cause smaller localized outbreaks; the C viruses are not important causes of disease in humans. Between pandemics, the viruses undergo constant, rapid evolution (a process called antigenic drift) in response to the pressures of human population immunity. Periodically, they undergo major evolutionary change by acquiring a new genome segment from another influenza virus (antigenic shift), effectively becoming a new subtype to which none, or very few, of the population is immune.
Influenza type A virus is the most frequent cause of influenza; this type occurs in numerous strains or subtypes that are differentiated mainly on the basis of two surface proteins, hemagglutinin (H) and neuraminidase (N). An antigenic shift in the influenza A virus can produce a pandemic affecting most of the world within a matter of months. For example, the influenza epidemic of 1918–19, the most destructive influenza outbreak in history and one of the most severe disease epidemics ever encountered, was caused by a subtype of influenza A known as H1N1. During this epidemic an estimated 25 million persons throughout the world died of the so-called Spanish flu, which was first widely reported in Spain but seems to have originated in the United States. Subsequent outbreaks of influenza have been much less severe. Influenza A subtype H2N2, or Asian flu, for instance, apparently began in East Asia early in 1957, and by midyear it had circled the globe. After 10 years of evolution that produced annual epidemics, the Asian flu disappeared in 1968, only to be replaced by a new influenza A subtype, H3N2. This virus, also known as Hong Kong flu, is still in circulation. In 1997 a type of avian influenza, or bird flu, virus broke out among domesticated poultry in Hong Kong and then infected a small number of people, killing some of them. This same virus, H5N1, reappeared among chicken flocks in Southeast Asia during the winter of 2003–04, again infecting some people fatally.
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