Three 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 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 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 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 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.^[2][3][4]
• 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 flu season, coinciding with the start of public school.^{[citation needed]} 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.^[5][6][7]

Research in guinea pigs has shown that the 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.^[2] 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.^[8]

Timing

Seasonal variation in deaths due to influenza or pneumonia in 122 U.S. cities, as a proportion of all causes.^[9]

In the United States, the flu season is considered October through May.^[10] It typically reaches an apex in February,^[11] with a seasonal baseli

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.^[8]

In the United States, the flu season is considered October through May.^[10] It typically reaches an apex in February,^[11] with a seasonal baseline varying between 6.1% and 7.7% of all deaths.^[9] In Australia, the flu season is considered May to October. It usually peaks in August.^[12]

Flu seasons also exist in the tropics and subtropics, with variability from region to region.^[13] In Hong Kong, which has a humid subtropical climate, the flu season runs from December to March, in the winter and early spring.^[14][15]

Flu vaccinations

Flu vaccinations are used to diminish the effects of the flu season and can lower an individual's risk of getting the flu by about half.^{Flu seasons also exist in the tropics and subtropics, with variability from region to region.[13] In Hong Kong, which has a humid subtropical climate, the flu season runs from December to March, in the winter and early spring.[14][15]}

Flu vaccinations are used to diminish the effects of the flu season and can lower an individual's risk of getting the flu by about half.^[16] 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) recommends two vaccine formulations every year; one for the Northern, and one for the Southern Hemisphere.^[17]

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.^[18]

The annually updated trivalent influenza vaccine consists of hemagglutinin (HA) surface glycoprotein components from influenza [18]

The annually updated trivalent influenza vaccine consists of hemagglutinin (HA) surface glycoprotein components from influenza H3N2, H1N1, and B influenza viruses.^[19] 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.^[20][21]

Medical conditions that compromise the immune system increase the risks from flu.

Diabetes

Millions of people have diabetes. When blood sugars are not well controlled, diabetics can quickly develop a wide range of complications. Diabetes results in elevated blood sugars in the body, and this environment allows viruses and bacteria to thrive.^{[citation needed]}

If blood sugars are poorly controlled, a mild flu can quickly turn severe, leading to hospitalization and even death. Uncontrolled blood sugars suppresses the immune systems and generally lead to more severe cases of th

Millions of people have diabetes. When blood sugars are not well controlled, diabetics can quickly develop a wide range of complications. Diabetes results in elevated blood sugars in the body, and this environment allows viruses and bacteria to thrive.^{[citation needed]}

If blood sugars are poorly controlled, a mild flu can quickly turn severe, leading to hospitalization and even death. Uncontrolled blood sugars suppresses the immune systems and generally lead to more s

If blood sugars are poorly controlled, a mild flu can quickly turn severe, leading to hospitalization and even death. Uncontrolled blood sugars suppresses the immune systems and generally lead to more severe cases of the common cold or influenza. Thus, it has been recommended that diabetics be vaccinated against flu, before the start of the flu season.^[22][23]

The CDC recommends that people with asthma and chronic obstructive pulmonary disease (COPD) be vaccinated against flu before the flu season. People with asthma can develop life-threatening complications from influenza and the common cold viruses. Some of these complications include pneumonias, acute bronchitis, and acute respiratory distress syndrome.^[24]

Each year flu related complications in the USA affect close to 100,000 asthmatics, and millions more are seen in the emergency room because of severe shortness of breath. T

Each year flu related complications in the USA affect close to 100,000 asthmatics, and millions more are seen in the emergency room because of severe shortness of breath. The CDC recommends that asthmatics are vaccinated between October and November, before the peak of the flu season. Flu vaccines take about two weeks to become effective.^[25]

People with cancer usually have a suppressed immune system. Moreover, many cancer patients undergo radiation therapy and potent immunosuppressive medications, which further suppresses the body's ability to fight off infections. Everyone with cancer is highly susceptible and is at risk for complications from flu. People with cancer or a history of cancer should receive the seasonal flu shot. Flu vaccination is also strict for lung cancer patients, as cancer leads to complications of pneumonia and bronchitis. People with cancer should not receive the nasal spray vaccine. The flu shot is made up of inactivated (killed) viruses, and the nasal spray vaccines are made up of live viruses. The flu shot is safer for those with a weakened immune system. Those who have received cancer treatment such as chemotherapy and/or radiation therapy within the last month, or have a blood or lymphatic form of cancer should call their doctor immediately if they suspect they may have flu.^[26]

HIV/AIDS