Virologists I’ve spoken to favor a phenomenon known as “viral chatter.” Viruses don’t just barge across the species barrier in one great rush. They tentatively make incursions–many of them spread across years. The SARS virus, for example, infected a few people before becoming a massive epidemic in 2003. During these incursions, animal viruses may acquire mutations that gradually prepare them to become good at spreading from person to person.
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