There are three different types of influenza viruses: A, B, and C. Type C rarely causes disease in humans. Type B does cause disease, but not epidemics. Only influenza A viruses cause epidemics or pandemics, an epidemic being a local or national outbreak, a pandemic a worldwide one. 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
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