When a new influenza virus emerges, it is highly competitive, even cannibalistic. It usually drives older types into extinction. This happens because infection stimulates the body’s immune system to generate all its defenses against all influenza viruses to which the body has ever been exposed. When older viruses attempt to infect someone, they cannot gain a foothold. They cease replicating. They die out. So, unlike practically every other known virus, only one type—one swarm or quasi species—of influenza virus dominates at any given time. This itself helps prepare the way for a new pandemic,
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