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Flu season

Flu season is an annually recurring time period characterized by the prevalence of an outbreak of influenza (flu). The season occurs during the cold half of the year in each hemisphere. It takes approximately two days to show symptoms. 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 three weeks to reach its pinnacle, and another three weeks to significantly diminish.[1]

"Influenza epidemic" redirects here. Not to be confused with Influenza pandemic.

Annually, about 3 to 5 million cases of severe illness and 290,000 to 650,000 deaths from seasonal flu occur worldwide.[2]

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 , preventing the body from effectively defending against respiratory virus infections.[3][4][5]

mucous membranes

Viruses are preserved in colder temperatures due to slower decomposition, so they linger longer on exposed surfaces (doorknobs, countertops, etc.).

production from Ultraviolet-B in the skin changes with the seasons and affects the immune system.[6][7][8]

Vitamin D

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 flu during the season, the viruses are actually transmitted throughout populations all year round.


Each annual flu season 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 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.[3] 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.[9]

Bird flu

Human flu

Horse flu

Dog flu

Freshers' flu

2012–2013 flu season

2017–2018 United States flu season

2019–2020 United States flu season

United States influenza statistics by flu season

— Contains about a dozen charts and graphs with extensive text

CDC U.S. influenza season summary with weekly updates

EU response to influenza

Health-EU portal

EU coordination on Pandemic (H1N1) 2009

European Commission - Public Health