A virus is a submicroscopic infectious agent that replicates only inside the living cells of an organism. Viruses can infect all types of life forms, from animals and plants to microorganisms, including bacteria and archaea. Since Dmitri Ivanovsky‘s 1892 article describing a non-bacterial pathogen infecting tobacco plants, and the discovery of the tobacco mosaic virus by Martinus Beijerinck in 1898, about 5,000 virus species have been described in detail, of the millions of types of viruses in the environment. Viruses are found in almost every ecosystem on Earth and are the most numerous type of biological entity. The study of viruses is known as virology, a subspeciality of microbiology.
By derivative work: MouagipAntigenicShift HiRes.png: National Institute of Allergy and Infectious Diseases (NIAID). – AntigenicShift HiRes.png, Public Domain, Link
An influenza pandemic is a global outbreak of a new influenza A virus that is very different from current and recently circulating human seasonal influenza A viruses. Influenza A viruses are constantly changing, making it possible on very rare occasions for non-human influenza viruses to change in such a way that they can infect people easily and spread efficiently from person to person.
Pigs experimentally infected with the strain of swine flu that caused the human pandemic of 2009–10 showed clinical signs of flu within four days, and the virus spread to other uninfected pigs housed with the infected ones and then to humans.