Transmission electron micrograph (TEM) depicts some of the ultrastructural details displayed by H3N2 influenza virions responsible for casing illness in Indiana and Pennsylvania in 2011. The cases of human infection with swine-origin H3N2 influenza resulted from existing influenza viruses exchanging genetic material through a process called "reassortment." Reassortment typically occurs when a host - animal or human - becomes infected with two or more different influenza viruses at the same time. This allows the influenza viruses to mix and exchange genetic information with each other, which in turn, can result in the emergence of new influenza viruses. Because pigs can be infected with and spread influenza viruses from birds, pigs and humans, they can represent a source for influenza virus reassortment to occur. This is particularly true in environments where humans, pigs and birds come into close contact with one another, such as farms.