Causes of Swine Flu

Swine flu is caused by the H1N1 strain of the influenza virus. The influenza virus constantly changes and mutates in order to escape the immune system of the animal it infects.

swine fluImage Credit: Iakov Filimonov /

Types of influenza virus

There are three major types of influenza viruses - influenza A, B and C. The influenza virus that causes swine flu is termed H1N1 2009 and belongs to the class influenza A.

The H1N1 viruses have been causing yearly flu outbreaks in humans for a long time, but the 2009 pandemic was caused by a variation in the usual H1N1 virus.

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).

Nomenclature of viruses

If detected, a variant 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.

Who does swine flu affect?

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, and most outbreaks occur during the late fall and winter months just like human seasonal flu infections.

Pigs are susceptible to bird flu, human flu and swine flu. The animals can 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 from a new 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.

Seasonal flu

The flu season occurs during winter. These outbreaks are called seasonal flu and typically about one per cent of the infected may die due to complications of flu like pneumonia.

Pandemic of 2009

Influenza A H1N1 2009 was first reported in Mexico and from there spread rapidly worldwide, with the World Health Organization (WHO) declaring a pandemic in June 2009.

The number of cases rose despite it being summer until October 2009 when they started to decline. By February 2010 the pandemic had killed 15,921 worldwide. The numbers fell thereafter mainly due to mass vaccinations. On the 10th of August 2010 the WHO declared the pandemic over.


Further Reading

Last Updated: Nov 4, 2022

Dr. Ananya Mandal

Written by

Dr. Ananya Mandal

Dr. Ananya Mandal is a doctor by profession, lecturer by vocation and a medical writer by passion. She specialized in Clinical Pharmacology after her bachelor's (MBBS). For her, health communication is not just writing complicated reviews for professionals but making medical knowledge understandable and available to the general public as well.


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