question: Why is the flu only around for a “season” instead of being around all of the time?
comment: It is around all the time but the flu mutates quite often, which is why there is no vaccination. Everyone adapts to the ‘current’ strain of the flu, and it disappears for a while, then it mutates, and everyone starts getting it again until they re-adapt. It’s always there, its just suffer from it in stages.
passages: However, seasonal changes in infection rates also occur in tropical regions, and in some countries these peaks of infection are seen mainly during the rainy season. Seasonal changes in contact rates from school terms, which are a major factor in other childhood diseases such as measles and pertussis, may also play a role in the flu. A combination of these small seasonal effects may be amplified by dynamical resonance with the endogenous disease cycles. H5N1 exhibits seasonality in both humans and birds.
   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.
  
  Section::::Flu vaccinations.
   Section::::Cost.
  
  The cost of a flu season in lives lost, medical expenses and economic impact can be severe. 
   BULLET::::- Viruses are preserved in colder temperatures due to slower decomposition, so they linger longer on exposed surfaces (doorknobs, countertops, "etc.").
  
  BULLET::::- 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. It is thought that the day care environment is perfect for the spread of illness.
  
  BULLET::::- Vitamin D production from Ultraviolet-B in the skin changes with the seasons and affects the immune system.
   Section::::Reception.:Reviews.
   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.
  
  Section::::Timing.
  
  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.
   Section::::Health conditions likely to cause complications.:HIV/AIDS.
   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.
   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.
   Medical conditions that compromise the immune system increase the risks from flu.
  
  Section::::Health conditions likely to cause complications.: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.
   Section::::Reception.
  
  Section::::Reception.:Ratings.
   Section::::Cultural references.
   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.
   Flu season
  
  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.
  
  Section::::Cause.
   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.
   An alternative hypothesis to explain seasonality in influenza infections is an effect of vitamin D levels on immunity to the virus. This idea was first proposed by Robert Edgar Hope-Simpson in 1965. He proposed that the cause of influenza epidemics during winter may be connected to seasonal fluctuations of vitamin D, which is produced in the skin under the influence of solar (or artificial) UV radiation. This could explain why influenza occurs mostly in winter and during the tropical rainy season, when people stay indoors, away from the sun, and their vitamin D levels fall.
  
  Section::::Epidemiology.:Epidemic and pandemic spread.
   Because the mortality rate of the H1N1 "swine flu" is lower than common flu strains, this number was actually lower in 2009. According to 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."
  
  Section::::Notable occurrences.
   These conditions can impair coughing, swallowing, clearing the airways, and in the worst cases, breathing. Therefore, they worsen the flu symptoms.
  
  Section::::Epidemiology.
  
  Section::::Epidemiology.:Seasonal variations.
  
  Influenza reaches peak prevalence in winter, and because the Northern and Southern Hemispheres have winter at different times of the year, there are actually two different flu seasons each year. This is why the World Health Organization (assisted by the National Influenza Centers) makes recommendations for two different vaccine formulations every year; one for the Northern, and one for the Southern Hemisphere.
   A long-standing puzzle has been why outbreaks of the flu occur seasonally rather than uniformly throughout the year. One possible explanation is that, because people are indoors more often during the winter, they are in close contact more often, and this promotes transmission from person to person. Increased travel due to the Northern Hemisphere winter holiday season may also play a role. Another factor is that cold temperatures lead to drier air, which may dehydrate mucus particles. Dry particles are lighter and can thus remain airborne for a longer period. The virus also survives longer on surfaces at colder temperatures and aerosol transmission of the virus is highest in cold environments (less than 5°C) with low relative humidity. The lower air humidity in winter seems to be the main cause of seasonal influenza transmission in temperate regions.
   It is recommended that asthmatics and COPD patients 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.
   This genus has one species, influenza B virus. Influenza B almost exclusively infects humans and is less common than influenza A. The only other animals known to be susceptible to influenza B infection are the seal and the ferret. This type of influenza mutates at a rate 2–3 times slower than type A and consequently is less genetically diverse, with only one influenza B serotype. As a result of this lack of antigenic diversity, a degree of immunity to influenza B is usually acquired at an early age. However, influenza B mutates enough that lasting immunity is not possible. This reduced rate of antigenic change, combined with its limited host range (inhibiting cross species antigenic shift), ensures that pandemics of influenza B do not occur.
   Due to the high mutation rate of the virus, a particular influenza vaccine usually confers protection for no more than a few years. Each year, the World Health Organization predicts which strains of the virus are most likely to be circulating in the next year (see Historical annual reformulations of the influenza vaccine), allowing pharmaceutical companies to develop vaccines that will provide the best immunity against these strains. The vaccine is reformulated each season for a few specific flu strains but does not include all the strains active in the world during that season. It takes about six months for the manufacturers to formulate and produce the millions of doses required to deal with the seasonal epidemics; occasionally, a new or overlooked strain becomes prominent during that time. It is also possible to get infected just before vaccination and get sick with the strain that the vaccine is supposed to prevent, as the vaccine takes about two weeks to become effective.
   The exact mechanism behind the seasonal nature of influenza outbreaks is unknown. Some proposed explanations are:
  
  BULLET::::- People are indoors more often during the winter, they are in close contact more often, and this promotes transmission from person to person.
  
  BULLET::::- 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.
  
  BULLET::::- Cold temperatures lead to drier air, which may dehydrate mucous membranes, preventing the body from effectively defending against respiratory virus infections.
   If a particular hemagglutinin or neuraminidase has been circulating outside the human population, then humans will lack immunity to this protein and an influenza pandemic may follow a host switch event, as seen in 1918, 1957, 1968 and 2009.
  
  After introduction into the human population, a lineage of influenza generally persists through antigenic drift, in which HA and NA continually accumulate mutations allowing viruses to infect hosts immune to earlier forms of the virus.
  
  These lineages of influenza show recurrent seasonal epidemics in temperate regions and less periodic transmission in the tropics.
   The United States Centers for Disease Control and Prevention (CDC) began counting the 2017–2018 "flu season" as October 2017, and by early February 2018, the epidemic was still widespread and increasing overall. By February 2018, the CDC said that the circulating virus strains included both B strains (Yamagata and Victoria), H1N1 and H3N2. On February 10, 2018, "Fortune" reported that influenza in the United States was killing up to 4,000 Americans a week, likely to far outstrip the rate of deaths in the 2009–2010 season. An expert was quoted saying that the main time of the flu that year hadn't "changed enough from previous seasons to be considered a novel strain." In the first week of February, deaths from influenza and pneumonia were responsible for one of every ten deaths in the US, with 4,064 from pneumonia or influenza recorded in the third week of 2018, according to CDC data. The CDC also reported 63 child deaths at that point, half of which were not considered medically high risk, and only about 20 percent who were vaccinated. Only two of those deaths were babies under six months old.
  
presuppositions: The flu is around for a season. Flu is only around for a season.
corrections: The flu is around all the time, but people adapt to it until it mutates again. It is around all the time.
raw_presuppositions: The flu is around for a season. Flu is only around for a season.
raw_corrections: The flu is around all the time, but people adapt to it until it mutates again. It is around all the time.