The 2009 flu pandemic is a global outbreak of a new strain of an influenza A virus subtype H1N1, referred to as the "novel H1N1"^[1] first identified in April 2009, and commonly called "swine flu". It is thought to be a reassortment of four known strains of influenza A virus^[2]: one endemic in (normally infecting) humans, one endemic in birds, and two endemic in pigs (swine). Transmission of the new strain is human-to-human, with cooked pork products safe to eat as the virus cannot be transmitted by eating foods.

The outbreak began in Mexico, with evidence that Mexico was already in the midst of an epidemic for months before the outbreak was recognized. Soon after, its government closed down most of Mexico City's public and private offices and facilities to help contain the spread. In early June, as the virus spread globally, the World Health Organization (WHO) declared the outbreak to be a pandemic, but also noted that most illnesses were of moderate severity. The virus has since spread to the Southern Hemisphere which entered its winter flu season, and to many less developed countries with limited healthcare systems. Because the virus was spreading with unprecedented speed, and many clinics were overwhelmed testing and treating patients, WHO stopped requiring countries to report all cases, but is still monitoring unusually large outbreaks.^[3][4]

The virus typically spread via coughing, sneezing or touching contaminated surfaces and then touching the nose or mouth. Symptoms, which could last up to a week, were similar to those of seasonal flu, and may have included fever, sneezes, sore throat, coughs, headache, and muscle or joint pains. The U.S. Centers for Disease Control and Prevention (CDC) noted that although this was a very serious virus, cases worldwide were usually mild, and most hospitalizations and deaths had been of persons that also had underlying conditions such as asthma, diabetes, obesity, heart disease, or a weakened immune system.^[5] As the virus spreads easily between people, through the air or surface contact, those who get the flu are recommended to stay home from school or work and avoid crowds to avoid spreading the infection further. Wearing facial masks is of limited benefit as they are designed for industrial use and do not filter out viral or other biological agents.^[6]

In an attempt to slow the spread of the illness, a number of countries, especially in Asia, quarantined airline passengers with flu symptoms, while some also pre-screened passengers. As of August 2009, WHO did not expect to have a full vaccine before the end of 2009, and vaccines available sooner would be limited and given first to healthcare workers, pregnant women, and other higher risk groups. Two or three injections would be required for maximum immunity from both the swine flu and seasonal flu. There was also concern if the new virus mutated further, it could become more virulent and less susceptible to any new vaccine.