H1N1

H1N1 is a subtype of influenza A virus characterized by the hemagglutinin 1 and neuraminidase 1 surface proteins. It has been responsible for notable outbreaks, including a pandemic virus, and continues to circulate as a component of seasonal influenza in some years.

Two major events shaped its history: the 1918 pandemic, caused by an H1N1 strain that spread worldwide with high mortality, and the 2009 pandemic, caused by a novel H1N1 strain of swine origin that rapidly disseminated. Since 2010, H1N1 has been included in seasonal influenza vaccines.

H1N1 spreads mainly through respiratory droplets and contact with contaminated surfaces. The incubation period is typically 1 to 4 days. Symptoms resemble other influenza illnesses and include fever, cough, sore throat, body aches, fatigue, and sometimes nausea or diarrhea, particularly in children. Severe disease is more likely in young children, older adults, pregnant people, and individuals with underlying health conditions.

Diagnosis is confirmed by laboratory testing, usually reverse transcription polymerase chain reaction (RT-PCR) on respiratory specimens. Rapid antigen tests exist but have lower sensitivity. Antiviral medications such as oseltamivir or zanamivir are most effective when started early, especially for high-risk patients or those with severe illness.

Prevention relies on annual vaccination, with the H1N1 component included in many modern seasonal flu vaccines. Public health surveillance tracks circulating strains; hygiene measures and antiviral stockpiles may be used during outbreaks. The term "swine flu" is commonly used in media but does not imply ongoing contact with pigs.