Swine flu is caused by the influenza virus. There are many types of influenza viruses.
The virus constantly changes and mutates in order to escape the immune system of the animal it infects.
The influenza virus that causes swine flu is termed H1N1 2009.
There are three major types of influenza viruses - influenza A, B and C.
The swine flu virus belongs to the class influenza A.
The H1N1 viruses have been causing yearly flu outbreaks in humans for a while.
However, the 2009 pandemic was caused by a variation in the usual H1N1 virus.
This is specifically called H1N1 2009 or swine flu.
This strain had not previously been found in pigs or humans. It carries a mixture of genes from human flu, pig flu (swine flu) and bird flu (avian flu).
A variant if detected is termed with an additional “v”. For example, if a H3N2 virus variation is detected in a person, it will be called “H3N2v” virus.
This nomenclature was devised in the January 6, 2012 Morbidity and Mortality Weekly Report from the Centers for Disease Prevention and Control. (1-4)
The Swine flu virus commonly affects pigs. It leads to flu like symptoms like fever, coughing (barking), discharge from the nose or eyes, sneezing, difficulty in breathing, red and watery eyes and refusal to eat.
Some pigs may be infected but do not appear sick. The virus rarely kills the pigs. Most outbreaks occur during the late fall and winter months just like human seasonal flu infections.
Further, pigs are susceptible to all three bird flu, human flu and swine flu. These animals may be infected with viruses from different species all at once.
Once this happens there is a potential that the viruses get mixed to create a new variation that could spread easily from person-to-person. When this happens to an influenza strain it is called antigenic shift.
Antigenic shift results when a new influenza A virus that people have never been exposed to, infects humans.
Consequently there is no immunity against the new variant virus.
This is what happened in 2009 when an influenza A H1N1 virus with swine, avian and human genes in a mixed pool of the H1N1 2009 strain led to the pandemic. (2, 3)
The flu season is during winter. These outbreaks are called seasonal flu.
About one per cent of the infected may die due to complications of flu like pneumonia.
Influenza A H1N1 2009 was first reported in Mexico. It spread rapidly from there worldwide.
The World Health Organization declared a pandemic in June 2009.
The number of cases rose despite it being summer till about October 2009 before they started to decline.
By February 2010 the pandemic had killed 15,921 worldwide. The numbers started to fall thereafter mainly due to mass vaccinations. On the 10th of August 2010 the WHO declared the pandemic over. (1)