Swine influenza in the context of "Orthomyxoviridae"

A zoonosis (/zoʊˈɒnəsɪs, ˌzoʊəˈnoʊsɪs/ ; plural zoonoses) or zoonotic disease is an infectious disease of humans caused by a pathogen (an infectious agent, such as a virus, bacterium, parasite, fungi, or prion) that can jump from a non-human vertebrate to a human. When humans infect non-humans, it is called reverse zoonosis or anthroponosis.

Major modern diseases such as Ebola and salmonellosis are zoonoses. HIV was a zoonotic disease transmitted to humans in the early part of the 20th century, though it has now evolved into a separate human-only disease. Human infection with animal influenza viruses is rare, as they do not transmit easily to or among humans. However, avian and swine influenza viruses in particular possess high zoonotic potential, and these occasionally recombine with human strains of the flu and can cause pandemics such as the 2009 swine flu. Zoonoses can be caused by a range of disease pathogens such as emergent viruses, bacteria, fungi and parasites; of 1,415 pathogens known to infect humans, 61% were zoonotic. Most human diseases originated in non-humans; however, only diseases that routinely involve non-human to human transmission, such as rabies, are considered direct zoonoses.

Influenza A virus subtype H1N1 (A/H1N1) is a subtype of influenza A virus (IAV). Some human-adapted strains of H1N1 are endemic in humans and are one cause of seasonal influenza (flu). Other strains of H1N1 are endemic in pigs (swine influenza) and in birds (avian influenza). Subtypes of IAV are defined by the combination of the antigenic hemagglutinin (H) and neuraminidase (N) proteins in the viral envelope; for example, "H1N1" designates an IAV subtype that has a type-1 H protein and a type-1 N protein.

All subtypes of IAV share a negative-sense, segmented RNA genome. Under rare circumstances, one strain of the virus can acquire genetic material through genetic reassortment from a different strain and thus evolve to acquire new characteristics, enabling it to evade host immunity and occasionally to jump from one species of host to another. Major outbreaks of H1N1 strains in humans include the 1918 Spanish flu pandemic, the 1977 Russian flu pandemic and the 2009 swine flu pandemic, all of which were caused by strains of A(H1N1) virus which are believed to have undergone genetic reassortment.