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Flu season is a regularly re-occurring time period characterized by the prevalence of outbreaks of influenza. 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. Individual cases of the flu however, usually only last a few days. In some countries such as Japan and China, infected persons sometimes wear a surgical mask out of respect for others.

Cause
Three virus families, Influenzavirus A, Influenzavirus B, and Influenzavirus C are the main infective agents that cause influenza. During periods of cooler temperature, influenza cases increase roughly tenfold or more, resulting in the flu season. Despite higher prevalence of disease cases during the season, these viruses are transmitted amongst people all year round. They do not disappear and reappear.

Each annual flu season is normally associated with a major influenzavirus subtype. The associated subtype changes each year, due to mutational changes amongst viral populations, as well as the development of immunological resistance to last year's strain from previous infection and vaccination.

Mechanism for seasonal nature of influenza
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.
 * Cold temperatures lead to drier air, which may dehydrate mucus, preventing the body from effectively expelling virus particles.
 * The virus may linger longer on exposed surfaces (doorknobs, countertops, etc.) in colder temperatures.
 * 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.

A recent research done by National Institute of Child Health and Human Development (NICHD) on influenza virus identified the virus as having "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.

Flu vaccinations
Flu vaccinations have been used to diminish the effects of the flu season. Since the Northern and Southern Hemisphere have winter at different times of the year, there are actually two flu seasons each year. Therefore, the World Health Organization (assisted by the National Influenza Centers) makes two vaccine formulations every year; one for the Northern, and one for the Southern Hemisphere.

According to the U.S. Department of Health, a growing number of large companies provide their employees with seasonal flu shots, either at a small cost to the employee or as a free service.

The annually updated trivalent influenza vaccine consists of hemagglutinin (HA) surface glycoprotein components from influenza H3N2, H1N1, and B influenza viruses. The dominant strain in January 2006 was H3N2. Measured resistance to the standard antiviral drugs amantadine and rimantadine in H3N2 has increased from 1% in 1994 to 12% in 2003 to 91% in 2005.

Cost
globalize The cost of a flu season in lives lost, medical expenses and economic impact can be severe. A recent study estimated that in the United States, annual inﬂuenza epidemics result in approximately 600,000 life-years lost, 3 million hospitalized days, and 30 million outpatient visits, resulting in medical costs of $10 billion annually. According to this study, lost earnings due to illness and loss of life amounted to over $15 billion annually and the total economic burden of annual inﬂuenza epidemics amounts to over $80 billion.