Patterns of infection

In temperate climates, fluA viral infection affecting the respiratory system. infectionInvasion by organisms that may be harmful, for example bacteria or parasites. spreads in epidemics every winter, lasting for around 8-10 weeks.[1] An epidemicA sudden outbreak of infection that affects a large proportion of a population. is a sudden outbreak of infectious disease that affects many people in a particular population.

In warmer climates - tropical and subtropical countries - fluA viral infection affecting the respiratory system. infections are seen throughout the year. Because of this, the impact of fluA viral infection affecting the respiratory system. is less obvious in these countries.[2]

It is not fully understood why fluA viral infection affecting the respiratory system. epidemics nearly always take place during the winter in temperate climates. Possible explanations include weather conditions that allow the virusA microbe that is only able to multiply within living cells. to survive more easily outside the airway of the infected person or animal, or it may be down to a change in behaviour - people spend a lot of time together indoors, which encourages the spread of infectionInvasion by organisms that may be harmful, for example bacteria or parasites..[3]

Research suggests that a fluA viral infection affecting the respiratory system. pandemicAn outbreak of infection that affects numerous people in different countries. has taken place roughly every 10-40 years for the past several hundred years

Emerging evidence also suggests that low levels of vitamin D - generated in the skin on exposure to sunlight - may play an important role in hostAn animal or plant that supports a parasite. susceptibility to influenzaA viral infection affecting the respiratory system. in the winter.[4]

Once in a while, an epidemicA sudden outbreak of infection that affects a large proportion of a population. takes place that is so widespread that huge numbers of people in different countries are affected. This is called a pandemicAn outbreak of infection that affects numerous people in different countries. . Research suggests that a fluA viral infection affecting the respiratory system. pandemicAn outbreak of infection that affects numerous people in different countries. has taken place roughly every 10-40 years for the past several hundred years.[2,5] Up to 50 per cent of the population can be infected in a single pandemicAn outbreak of infection that affects numerous people in different countries. year.[5]

In June 2009 the World Health Organization declared the most recent pandemicAn outbreak of infection that affects numerous people in different countries. , swine fluA viral infection affecting the respiratory system. - a type of influenzaA viral infection affecting the respiratory system. A virusA microbe that is only able to multiply within living cells. called H1N1. The name refers to the proteins on the surface of the virusA microbe that is only able to multiply within living cells., which differ depending on the subtypeA group within a group..

At that time, there were almost 30,000 confirmed cases of H1N1 influenzaA viral infection affecting the respiratory system. reported in 74 countries.[6]  This is the first pandemicAn outbreak of infection that affects numerous people in different countries. reported in 41 years.[7]

The last known pandemicAn outbreak of infection that affects numerous people in different countries. was the Hong Kong fluA viral infection affecting the respiratory system. pandemicAn outbreak of infection that affects numerous people in different countries. in 1968, which was caused by the H3N2 virusA microbe that is only able to multiply within living cells.. The most severe recorded pandemicAn outbreak of infection that affects numerous people in different countries. was that of 1918, the Spanish fluA viral infection affecting the respiratory system. pandemicAn outbreak of infection that affects numerous people in different countries. , which was caused by another H1N1 influenzaA viral infection affecting the respiratory system. virusA microbe that is only able to multiply within living cells..[8]

References: 
  1. DM Fleming. Influenza diagnosisThe process of determining which condition a patient may have. and treatment: A view from clinical practice. Phil Trans R Soc Lond. 2001;356:1933-43.
  2. Hampson AW and Mackenzie JS. The influenzaA viral infection affecting the respiratory system. virusesMicrobes that are only able to multiply within living cells.. MJA. 2006;185(10 Suppl):S39-43.
  3. Zambon MC. Epidemiology and pathogenesis of influenzaA viral infection affecting the respiratory system.. Journal of Antimicrobial Chemotherapy. 1999;44:3-9.
  4. Yamshchikov AV, Desai NS, Blumberg HM, Ziegler TR, Tangpricha V.
  5. Vitamin D for treatment and prevention of infectious diseases: a systematic review of randomized controlled trials. Endocr Pract. 2009 Jul-Aug;15(5):438-49.
  6. Taubenberger JK and Morens DM. The pathology of influenzaA viral infection affecting the respiratory system. virusA microbe that is only able to multiply within living cells. infections. Annu Rev Pathol. 2008;3:499-522.
  7. World Health Organization. Link
  8. Nature. Link
  9. Nature. Link