Influenza

Influenza, also known as the flu, or grippe, is 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.