Influenza viruses did not originate in humans. Their natural home is in wild aquatic birds, and many more variants of influenza viruses exist in birds than in humans. But the disease is considerably different in birds and humans. In birds, the virus infects the gastrointestinal tract. Bird droppings contain large amounts of virus, and infectious virus can contaminate cold lakes and other water supplies. Massive exposure to an avian virus can infect man directly, but an avian virus cannot go from person to person. It cannot, that is, unless it first changes, unless it first adapts to humans.
