An influenza pandemic is an epidemic of an influenza virus that spreads on a worldwide scale and infects a large proportion of the human population. In contrast to the regular seasonal epidemics of influenza, these pandemics occur irregularly, with the 1918 Spanish flu the most serious pandemic in recent history. Pandemics can cause high levels of mortality, with the Spanish influenza estimated as being responsible for the deaths of over 50 million people. There have been about three influenza pandemics in each century for the last 300 years. The most recent one was the 2009 flu pandemic.

Influenza pandemics occur when a new strain of the influenza virus is transmitted to humans from another animal species. Species that are thought to be important in the emergence of new human strains are pigs, chickens and ducks. These novel strains are unaffected by any immunity people may have to older strains of human influenza and can therefore spread extremely rapidly and infect very large numbers of people. Influenza A viruses can occasionally be transmitted from wild birds to other species causing outbreaks in domestic poultry and may give rise to human influenza pandemics.

The World Health Organization (WHO) has produced a six-stage classification that describes the process by which a novel influenza virus moves from the first few infections in humans through to a pandemic. This starts with the virus mostly infecting animals, with a few cases where animals infect people, then moves through the stage where the virus begins to spread directly between people, and ends with a pandemic when infections from the new virus have spread worldwide.

The 2009 flu pandemic or swine flu was an influenza pandemic that lasted from early 2009 to late 2010, and the second of the two pandemics involving H1N1 influenza virus (the first of them being the 1918-20 Spanish flu pandemic), albeit in a new version. First described in April 2009, the virus appeared to be a new strain of H1N1 which resulted when a previous triple reassortment of bird, swine and human flu viruses further combined with a Eurasian pig flu virus, leading to the term "swine flu". Unlike most strains of influenza, H1N1 does not disproportionately infect adults older than 60 years; this was an unusual and characteristic feature of the H1N1 pandemic".

Some pandemics are relatively minor such as the one in 1957 called "Asian flu" (1 – 4 million dead, depending on source). Others have a higher Pandemic Severity Index whose severity warrants more comprehensive social isolation measures.

The 1918 pandemic killed tens of millions and sickened hundreds of millions; the loss of this many people in the population caused upheaval and psychological damage to many people. There were not enough doctors, hospital rooms, or medical supplies for the living as they contacted the disease. Dead bodies were often left unburied as few people were available to deal with them. There can be great social disruption as well as a sense of fear. Efforts to deal with pandemics can leave a great deal to be desired because of human selfishness, lack of trust, illegal behavior, and ignorance. For example in the 1918 pandemic: "This horrific disconnect between reassurances and reality destroyed the credibility of those in authority. People felt they had no one to turn to, no one to rely on, no one to trust."

Flu pandemics typically come in waves. The 1889–1890 and 1918–1919 flu pandemics each came in three or four waves of increasing lethality. But within a wave, mortality was greater at the beginning of the wave.