Some virologists theorize that pigs provide a perfect “mixing bowl,” because the sialic-acid receptors on their cells can bind to both bird and human viruses. Whenever an avian virus infects swine at the same time that a human virus does, reassortment of the two viruses can occur. And an entirely new virus can emerge that can infect man. In 1918 veterinarians noted outbreaks of influenza in pigs and other mammals, and pigs today still get influenza from a direct descendant of the 1918 virus. But it is not clear whether pigs caught the disease from man or man caught it from pigs.
