The swine flu is likely a descendant of the infamous "Spanish flu" that caused a devastating pandemic in humans in 1918–1919. In less than a year, that pandemic killed more an estimated 50 million people worldwide. Descendants of this virus have persisted in pigs; they probably circulated in humans until the appearance of the Asian flu in 1957, and reemerged in 1977. Direct transmission from pigs to humans is rare, with 12 cases in the U.S. since 2005.
The flu virus is perhaps the trickiest known to medical science; it constantly changes form to elude the protective antibodies that the body has developed in response to previous exposures to influenza or to influenza vaccines.
Swine influenza (also swine flu) refers to influenza caused by any strain of the influenza virus endemic in pigs (swine). Strains endemic in swine are called swine influenza virus (SIV).
In humans, the symptoms of swine flu are similar to those of influenza and of influenza-like illness in general, namely chills, fever, sore throat, muscle pains, severe headache, coughing, weakness and general discomfort. The strain responsible for the 2009 swine flu outbreak in most cases causes only mild symptoms and the infected person makes a full recovery without requiring medical attention and without the use of antiviral medicines.
Swine flu is common in swine and rare in humans. People who work with swine, especially people with intense exposures, are at risk of catching swine influenza if the swine carry a strain able to infect humans. However, these strains rarely are able to pass from human to human. Rarely, SIV mutates into a form able to pass easily from human to human. The strain responsible for the 2009 swine flu outbreak is believed to have undergone such a mutation. This virus is named swine flu because one of its surface proteins is similar to viruses that usually infects pigs, but this strain is spreading in people and it is unknown if it infects pigs.