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Swine flu is a respiratory disease, caused by a strain of the influenza type A virus known as H1N1. Novel H1N1 (referred to as swine flu early on) is a new influenza virus causing illness in people. This new virus was first detected in people in the United States in April 2009.

Swine Flu virus is genetically different from the fully human H1N1 seasonal influenza virus that has been circulating globally for the past few years. It contains DNA that is typical to avian, swine and human viruses, including elements from European and Asian swine viruses. It has two genes from flu viruses that normally circulate in pigs in Europe and Asia and bird (avian) genes and human genes. Scientists call this a "quadruple reassortant" virus. The flu viruses from humans and pigs have swapped genetic components with each other, and evolved into a completely new variety Hence the absence of any concrete treatment plan for it.

Although the strain may have originated in pigs, it is now a wholly human disease and is being transmitted from person-to-person. Illness with the new H1N1 virus has ranged from mild to severe. While most people who have been sick have recovered without needing medical treatment, hospitalizations and deaths from infection with this virus have occurred.