RNA viruses mutate faster than almost any other sort of creature on Earth. In fact, Burke wrote, they mutate about a thousand times faster than animals: roughly one mistake in every ten thousand bases of genome. Although the genomes of RNA viruses are relatively short, only a few thousand or twenty thousand or thirty thousand bases (compared to three billion bases for the human genome), that error rate is enough to put at least one mutation into every new virion (every viral particle) of the typical RNA virus. Result: each of those new virions is likely to be different, by at least one
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