"Different RNA viruses mutate at different rates as well. A few mutate so rapidly that virologists consider them not so much a population of copies of the same virus as what they call a 'quasi species' or a 'mutant swarm.'
"These mutant swarms contain trillions and trillions of closely related but different viruses. Even the viruses produced from a single cell will include many different versions of themselves, and the swarm as a whole will routinely contain almost every possible permutation of its genetic code."
—John M. Barry, The Great Influenza (NY: Penguin Books, 2004), 105