Although the disease that caused the worldwide epidemic was called ''Spanish flu,'' the virus apparently is a mutation that evolved in American pigs and was spread around the globe by U.S. troops mobilized for World War I, said Taubenberger. Science is publishing the study today. Army doctors in 1918 conducted autopsies on some of the 43,000 servicemen killed by the flu and preserved some specimens in formaldehyde and wax. The hemagglutinin gene matches closest to swine influenza viruses, showing that this virus came into humans from pigs,'' said Taubenberger. The finding supports a widespread theory that flu viruses from swine are the most virulent for humans.
Most experts believe that flu viruses reside harmlessly in birds, where they are genetically stable.
Occasionally, a virus from birds will infect pigs. The swine immune system attacks the virus, forcing it to change genetically to survive. The result is a new virus. When this new bug is spread to humans, it can be devastating, said Taubenberger.