Seasonal influenza

Flu season is an annually recurring time period characterized by the prevalence of outbreaks of influenza (flu). The season occurs during the cold half of the year in each hemisphere. Influenza activity can sometimes be predicted and even tracked geographically. While the beginning of major flu activity in each season varies by location, in any specific location these minor epidemics usually take about 3 weeks to peak and another 3 weeks to significantly diminish immediately.

Three virus families, Influenzavirus A, B, and C are the main infective agents that cause influenza. During periods of cooler temperature, influenza cases increase roughly tenfold or more. Despite higher incidence of manifestations of the flu during the season, the viruses are actually transmitted throughout populations all year round.

Each annual flu season is normally associated with a major influenzavirus subtype. The associated subtype changes each year, due to development of immunological resistance to a previous year's strain (through exposure and vaccinations), and mutational changes in previously dormant viruses strains.

The exact mechanism behind the seasonal nature of influenza outbreaks is unknown. Some proposed explanations are:

Research in guinea pigs has shown that the aerosol transmission of the virus is enhanced when the air is cold and dry. The dependence on aridity appears to be due to degradation of the virus particles in moist air, while the dependence on cold appears to be due to infected hosts shedding the virus for a longer period of time. The researchers did not find that the cold impaired the immune response of the guinea pigs to the virus.

Research done by National Institute of Child Health and Human Development (NICHD) in 2008 found that the influenza virus has a "butter-like coating". The coating melts when it enters the respiratory tract. In the winter, the coating becomes a hardened shell; therefore, it can survive in the cold weather similar to a spore. In the summer, the coating melts before the virus reaches the respiratory tract.

In the United States, the flu season is considered October through May. It usually peaks in February. In Australia, the flu season is considered May to October. It usually peaks in August.

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