[11] The 1918 flu was an unusually severe and deadly strain of H1N1[12] avian influenza, which killed from 17[13] to 50 or more million people worldwide over about a year in 1918 and 1920.

The 1918 flu caused an abnormally high number of deaths, possibly due to it provoking a cytokine storm in the body.

[17] The term "Spanish" flu was coined because Spain was at the time the only European country where the press were printing reports of the outbreak, which had killed thousands in the armies fighting World War I (1914–1918).

[18] In 1976, a novel swine influenza A (H1N1) caused severe respiratory illness in 13 soldiers, with one death at Fort Dix, New Jersey.

[30] While viruses with this genetic makeup had not previously been found to be circulating in humans or pigs, there is no formal national surveillance system to determine what viruses are circulating in pigs in the U.S.[31] In April 2009, an outbreak of influenza-like illness (ILI) occurred in Mexico and then in the United States;[32] the CDC reported seven cases of novel A/H1N1 influenza and promptly shared the genetic sequences on the GISAID database.

[36] The outbreak had been predicted a year earlier by noticing the increasing number of replikins, a type of peptide, found in the virus.

[39] The President's declaration caused many U.S. employers to take actions to help stem the spread of the swine flu and to accommodate employees and / or workflow which may have been impacted by an outbreak.

The study authors suggest physician evaluation via contrast enhanced CT scans for the presence of pulmonary emboli when caring for patients diagnosed with respiratory complications from a "severe" case of the H1N1 flu.

The type of embolic events caused by H1N1 infection are summarized in a 2010 review by Dimitroulis Ioannis et al.[42] The 21 March 2010 worldwide update, by the U.N.'s World Health Organization (WHO), states that "213 countries and overseas territories/communities have reported laboratory confirmed cases of pandemic influenza H1N1 2009, including at least 16,931 deaths.

[53] On 17 March 2014, three cases were confirmed with a possible fourth awaiting results occurring at the Centre for Addiction and Mental Health in Toronto, Ontario, Canada.

[62] The Ministry of Health and Sports of Myanmar sent an official request to WHO to provide help to control the virus; and also mentioned that the government would be seeking international assistance, including from the UN, China and the United States.

[68] A 2020 peer-reviewed paper from the Proceedings of the National Academy of Sciences (PNAS) stated that "G4 EA H1N1 viruses possess all the essential hallmarks of being highly adapted to infect humans ...

Controlling the prevailing G4 EA H1N1 viruses in pigs and close monitoring of swine working populations should be promptly implemented.

[73] They also said the infected workers "did not show flu symptoms and the test sample is not representative of the pig population in China".

[71] The US Centers for Disease Control and Prevention (CDC) said the study suggested that human infection by the G4 virus is more common than it was thought to be.

[68] Both the European Centre for Disease Prevention and Control (ECDC)[74] and the US CDC[68] stated that, like all flu viruses with pandemic potential, the variant is a concern that will be monitored.

[74] The ECDC stated that "the most important intervention in preparing for the pandemic potential of influenza viruses is the development and use of human vaccines ...".

[74] Health officials (including Anthony Fauci) have said that the virus should be monitored, particularly among those in close contact with pigs, but it is not an immediate threat.

[76] Pregnant women who contract the H1N1 infection are at greater risk of developing complications because of hormonal changes, physical changes and changes to their immune system to accommodate the growing fetus.

[80] A study conducted in Japan during the 2009 H1N1 pandemic concluded that infants exposed to either oseltamivir or zanamivir had no short term adverse effects.

[81] Both amantadine and rimantadine have been found to be teratogenic and embryotoxic (malformations and toxic effects on the embryo) when given at high doses in animal studies.