Influenza-like disease was recognized clinically among pigs in the United States during the late summer and fall of 1918, and recent evidence clearly indicates that the 1918 human virus and the 1930s swine viruses are closely related. However, it remains unclear whether these viruses appeared first in people and then spread to pigs, or vice versa. H1N1 influenza viruses have continued to infect pigs in the United States ever since the 1930s. In addition, H3 subtype viruses have infected pigs in Europe, Asia and, as of the fall of 1998, in the U.S.