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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.〔(National Institute of Allergy and Infectious Diseases (NIAID) Factsheet )〕 The 2012–13 flu season was particularly harsh in the United States, where the majority of states were reporting high rates of influenza-like illness. The Centers for Disease Control reported that the available flu vaccine was 60% effective.〔 It further recommended that all persons over age 6 months get the vaccine.〔〔 ==Cause== (詳細は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 unclear. Some proposed explanations are: *People are indoors more often during the winter, they are in close contact more often, and this promotes transmission from person to person. *A seasonal decline in the amount of ultraviolet radiation may reduce the likelihood of the virus being damaged or killed by direct radiation damage or indirect effects (i. e. ozone concentration) increasing the probability of infection. *Cold temperatures lead to drier air, which may dehydrate mucous membranes, preventing the body from effectively defending against respiratory virus infections.〔 *The virus may linger longer on exposed surfaces (doorknobs, countertops, etc.) in colder temperatures. *In nations where children do not go to school in the summer, there is a more pronounced beginning to flu season, coinciding with the start of public school. It is thought that the day care environment is perfect for the spread of illness. *Vitamin D production from Ultraviolet-B in the skin changes with the seasons and affects the immune system. 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. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Flu season」の詳細全文を読む スポンサード リンク
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