used to be thought that all viruses cause disease—hence the Peter Medawar quotation—but we now know that most viruses infect only bacterial cells and have no effect on us at all. Of the hundreds of thousands of viruses reasonably supposed to exist, just 586 species are known to infect mammals, and of these only 263 affect humans. We know very little about most other, nonpathogenic viruses because only the ones that cause disease tend to get studied. In 1986,
