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Flu season is an annually recurring time period characterized by the prevalence of outbreaks of influenza (flu). 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.
The 2012–13 flu season was particularly harsh in the United States, where the majority of states were reporting high rates of influenza-like illness. The Centers for Disease Control reported that the available flu vaccine was 60% effective. It further recommended that all persons over age 6 months get the vaccine. The 2013-14 flu season is currently in progress and is above epidemic levels in many countries, including Canada, the United States, Greece and Finland.
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 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 unclear. 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. 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 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.
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.
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.
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.
Medical conditions that compromise the immune system increase the risks from flu.
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.
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 are vaccinated against flu, before the start of the flu season.
It is recommended that asthmatics be vaccinated against flu before the flu season. People with asthma can develop life-threatening complications from influenza and the common cold virus. Some of these complications include pneumonias and acute respiratory distress syndrome.
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. It is recommended that asthmatics are vaccinated between October and November, before the peak of the flu season. Flu vaccine works by boosting the body's immune system and takes about 2 weeks to become effective.
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. 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. 
Individuals who acquire the human immune deficiency virus (HIV) are very prone to a variety of infections. HIV has a tremendous capacity to destroy the body's immune system and this makes one prone to not only viral infections but also bacterial fungal and protozoa disorders. People with HIV are at an increased risk of serious flu-related complications. Many reports have shown that individuals with HIV can develop serious pneumonias which need hospitalization and aggressive antibiotic therapy. Moreover, people with HIV have a longer flu season and are at a high risk of death. Vaccination with the flu shot has been shown to boost the immune system and protect against the seasonal flu in some patients with HIV; individuals who have HIV should only get vaccinated with the inactivated influenza vaccine. Any HIV patient who has been exposed to other people with influenza should see a physician to determine if there is a need for anti viral medications.
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. Also, in the US the flu season usually accounts for 200,000 hospitalizations and 41,000 deaths.
Because the mortality rate of the H1N1 "swine flu" is lower than common flu strains, this number was actually lower in 2009. According the an article in Clinical Infectious Diseases, published in 2011, the estimated health burden of 2009 Pandemic Influenza A (H1N1), between April 2009 to April 2010, was "approximately 60.8 million cases (range: 43.3–89.3 million), 274,304 hospitalizations (195,086–402,719), and 12,469 deaths (8,868–18,306)" "in the United States due to pH1N1."