Swine influenza is an infection caused by any of several types of swine influenza viruses. Swine influenza virus (SIV) or swine-origin influenza virus (S-OIV) refers to any strain of the influenza family of viruses that is endemic in pigs. As of 2009, identified SIV strains include influenza C and the subtypes of influenza A known as H1N1, H1N2, H2N1, H3N1, H3N2, and H2N3.

The swine influenza virus is common throughout pig populations worldwide. Transmission of the virus from pigs to humans is rare and does not always lead to human illness, often resulting only in the production of antibodies in the blood. If transmission causes human illness, it is called a zoonotic swine flu. People with regular exposure to pigs are at increased risk of swine flu infections.

Around the mid-20th century, the identification of influenza subtypes was made possible, allowing accurate diagnosis of transmission to humans. Since then, only 50 such transmissions have been confirmed. These strains of swine flu rarely pass from human to human. Symptoms of zoonotic swine flu in humans are similar to those of influenza and influenza-like illness and include chills, fever, sore throat, muscle pains, severe headache, coughing, weakness, shortness of breath, and general discomfort.

It is estimated that, in the 2009 flu pandemic, 11–21% of the then global population (of about 6.8 billion), equivalent to around 700 million to 1.4 billion people, contracted the illness—more, in absolute terms, than the Spanish flu pandemic.^{[citation needed]} There were 18,449 confirmed fatalities. However, in a 2012 study, the CDC estimated more than 284,000 possible fatalities worldwide, with numbers ranging from 150,000 to 575,000. In August 2010, the World Health Organization declared the swine flu pandemic officially over.

Subsequent cases of swine flu were reported in India in 2015, with over 31,156 positive test cases and 1,841 deaths.

In pigs, a swine influenza infection produces fever, lethargy, discharge from the nose or eyes, sneezing, coughing, difficulty breathing, eye redness or inflammation, and decreased appetite. In some cases, the infection can cause miscarriage. However, infected pigs may not exhibit any symptoms. Although mortality is usually low (around 1–4%), the virus can cause weight loss and poor growth, in turn causing economic loss to farmers. Infected pigs can lose up to 12 pounds of body weight over a three- to four-week period. Influenza A is responsible for infecting swine and was first identified in 1918. Because both avian and mammalian influenza viruses can bind to receptors in pigs, pigs have often been seen as "mixing vessels", facilitating the evolution of strains that can be passed on to other mammals, such as humans.

Direct transmission of a swine flu virus from pigs to humans is possible (zoonotic swine flu). Fifty cases are known to have occurred since the first report in medical literature in 1958, which have resulted in a total of six deaths. Of these six people, one was pregnant, one had leukemia, one had Hodgkin's lymphoma, and two were known to be previously healthy. No medical history was reported for the remaining case The true rate of infection may be higher, as most cases only cause a very mild disease and may never be reported or diagnosed.

According to the United States Centers for Disease Control and Prevention (CDC), in humans the symptoms of the 2009 "swine flu" H1N1 virus are similar to influenza and influenza-like illness. Symptoms include fever, cough, sore throat, watery eyes, body aches, shortness of breath, headache, weight loss, chills, sneezing, runny nose, coughing, dizziness, abdominal pain, lack of appetite, and fatigue. During the 2009 outbreak, an elevated percentage of patients reporting diarrhea and vomiting.