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Wild birds carry a type of influenza A virus, called avian influenza virus, in their intestines, and usually do not get ill from it. However, avian influenza virus can make domesticated birds, including chickens, turkeys, and ducks, quite ill and can lead to death. Although avian influenza virus is chiefly found in birds, infection in humans from contact with infected poultry has been reported since 1996. A particular subtype of avian influenza A virus, H5N1, is highly contagious and deadly among birds. In 1997 in Hong Kong, an outbreak of avian influenza H5N1 occurred not only in poultry, but also in 18 humans, six of whom died. In subsequent infections of avian influenza H5N1 in humans, more than half of those infected with the virus have died. In contrast to seasonal influenza, most cases of avian influenza H5N1 have occurred in young adults and healthy children who have been exposed to infected poultry, or surfaces contaminated with H5N1 virus.

Although transmission of avian influenza H5N1 from human to human is rare, inefficient, and not sustained, there is concern that the H5N1 virus could adapt and acquire the ability for sustained transmission in the human population. If the H5N1 virus could gain the ability to transmit easily from person to person, a global influenza pandemic could occur. As of June 2020, there were a cumulative 861 cases of human cases of H5N1 reported to the World Health Organization, resulting in 455 deaths. A number of vaccines are currently available for H5N1, the first approved in 2007 and the latest in 2020. Given that the H5N1 virus continually mutates, the best protection for new strains of H5N1 will depend on a vaccine specifically produced for any future virus strain. The H5N1 virus is resistant to the adamantanes, but sensitive to the neuraminidase inhibitors (e.g., oseltamivir, zanamivir) [5].

In April 2009, a novel influenza A (H1N1) virus caused respiratory illness across North America and many areas of the world. The 2009 influenza A (H1N1), while similar to other H1N1 viruses, was genetically and antigenically distinct. Influenza morbidity caused by the 2009 pandemic influenza A (H1N1) remained above seasonal baselines throughout spring and summer and was the first pandemic since 1968. Data from epidemiologic studies conducted during the 2009 influenza A (H1N1) pandemic indicate that the risk for influenza complications among adults aged 19‐64 years who had 2009 pandemic influenza A (H1N1) was greater than typically occurs for seasonal influenza [6].

Avian influenza A (H7N9) virus is a subtype of influenza viruses not previously seen in either animals or people until it was found in China in February 2013. Since its discovery, infections in both humans and birds have been identified. While it has not been reported in birds outside China, its low pathogenicity in birds makes it difficult to identify international spread. Most of the cases of human H7N9 virus infections have reported recent exposure to live poultry or potentially contaminated environments, especially markets where live birds have been sold. Since its discovery, there have been 1,568 confirmed cases and 616 deaths due to H7N9 [7]. The disease is of concern because most people who become infected have become severely ill. This virus does not appear to transmit easily from person to person, and sustained human‐to‐human transmission has not been reported. Asymptomatic and mild infections have been detected, but the underlying rate of such infections is not well understood [8]. There is no current vaccine for H7N9. As with H5N1, neuraminidase inhibitors are effective against H7N9, but adamantanes are not [9].