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.

Ecological and taxonomic variation among human RNA viruses.

Only a minority of RNA viruses that can infect humans are capable of spreading in human populations independently of a zoonotic reservoir. This is especially true of vector-borne RNA viruses; the majority of these are not transmissible (via the vector) between humans at all. Understanding the biology underlying this observation will help us evaluate the public health risk associated with novel vector-borne RNA viruses.

The diversity of human RNA viruses.

We still cannot answer the very basic question "how many kinds of RNA viruses are there?" even for those that infect humans. It is often suggested that there remains a large number of viruses in humans that we have not yet discovered or recognised, and that there is a much larger and rapidly evolving pool of potential new viruses in mammalian and avian reservoirs that humans are continually being exposed to. However, a careful examination of discovery rates of new human RNA virus species, genera and families challenges this view, raising the possibility that the diversity is much more limited.