- Last Update On : 2013-01-20
The term 'flu' is often used for any febrile respiratory illness with systemic symptoms that may be caused be a myriad of bacterial or viral agents as well as influenza viruses. However, true influenza is an acute infectious disease caused by a member of the orthomyxovirus family, which includes influenza virus A, B and C. Influenza outbreaks usually occur in the winter in temperate climates. In the United States, the flu season usually starts in October or November and peaks between December and March.
Major outbreaks of influenza are associated with influenza virus type A or B. Influenza A infects birds, humans, swine, horses, seals and dogs. Influenza A is responsible for frequent, usually annual outbreaks or epidemics of varying intensity, and occasional pandemics, whereas influenza B causes outbreaks every two to four years. Influenza B viruses cause the same spectrum of disease as influenza A. However, influenza B viruses do not cause pandemics, possibly because they p