Under a plate magnification of 37,800X, this colorized transmission electron micrograph (TEM) depicted a strain of swine flu, the A/New Jersey/76 (Hsw1N1) virus, while in the virus? first developmental passage through a chicken egg. Swine Influenza (swine flu) is a respiratory disease of pigs caused by type A influenza that regularly cause outbreaks of influenza among pigs. Swine flu viruses cause high levels of illness and low death rates among pigs. Swine influenza viruses may circulate in swine throughout the year, but most outbreaks among swine herds occur during the late fall and winter months similar to humans. The classical swine flu virus (an influenza type A H1N1 virus) was first isolated from a pig in 1930.