Three virus families, Influenza virus 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 the higher incidence of manifestations of the<unk>  during the<unk> , the viruses are actually transmitted throughout populations all year round. Each annual<unk>  is normally associated with a major influenza virus sub type. The associated sub type 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<unk> al nature of influenza outbreaks is unknown. 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<unk> al 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. * Viruses are preserved in colder temperatures due to slower decomposition, so they linger longer on exposed surfaces (doorknobs, countertops, etc.). * In nations where children do not go to school in the summer, there is a more pronounced beginning to<unk> , 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<unk> s 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 the 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.
Flu season
Pandemic
Influenza pandemic
Timeline of influenza
PLOS Pathogens
Bedpan
Third plague pandemic
First plague pandemic