RNA Viruses: A Case Study of the Biology of Emerging Infectious Diseases.

There are 180 currently recognized species of RNA virus that can infect humans, and on average, 2 new species are added every year. RNA viruses are routinely exchanged between humans and other hosts (particularly other mammals and sometimes birds) over both epidemiological and evolutionary time: 89% of human-infective species are considered zoonotic and many of the remainder have zoonotic origins. Some viruses that have crossed the species barrier into humans have persisted and become human-adapted viruses, as exemplified by the emergence of HIV-1. Most, however, have remained as zoonoses, and a substantial number have apparently disappeared again. We still know relatively little about what determines whether a virus is able to infect, transmit from, and cause disease in humans, but there is evidence that factors such as host range, cell receptor usage, tissue tropisms, and transmission route all play a role. Although systematic surveillance for potential new human viruses in nonhuman hosts would be enormously challenging, we can reasonably aspire to much better knowledge of the diversity of mammalian and avian RNA viruses than exists at present.