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. Moreover, there is some evidence that the cast of human viruses is dynamic, with existing viruses disappearing (at least from humans) and new viruses appearing (perhaps evolving) over timescales of decades. Most of these new viruses, however, remain rare; only a small (but highly significant) minority are capable of spreading extensively through human populations.