<P> 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 . </P> <P> 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 . </P> <P> 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 . </P> <P> Flu seasons also exist in the tropics and subtropics, but are usually less sharply defined . In Hong Kong, which has a humid subtropical climate, the flu season runs from December to March, in the winter and early spring . </P>

When does the flu season begin and end