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

Abstract

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.

Figure 1

A representation of the pathogen pyramid. Each level of the pyramid represents a different degree of interaction between a virus and a human host. Level 1 corresponds to exposure of humans, level 2 to the ability to infect humans, level 3 to the ability to transmit from one human to another, and level 4 to the ability to cause epidemics or persist as an endemic infection. Arrows indicate pathways that viruses may take to reach each level. For example, a level 4 virus may arrive at that state directly, simply by exposure to the virus from a nonhuman reservoir. This is known as an “off-the-shelf” virus. Alternatively, it may initially enter the population as a level 2 or 3 virus—not capable of sustained transmission—but evolve the ability to transmit between humans at a sufficiently high rate to persist within a human population. This is known as a “tailor-made” virus. Adapted from reference . doi:10.1128/microbiolspec.OH-0001-2012.f1

Figure 2

All currently recognized human-infective RNA viruses categorized with respect to their ability to infect and transmit from humans (levels 2, 3, and 4 of the virus pyramid—see Fig. 1) and distinguished in terms of transmission route (green for vector-borne transmission, blue for other routes) and nature of diagnostic evidence (the viruses not in boldface type have only been reported in humans using serology-based methods). doi:10.1128/microbiolspec.OH-0001-2012.f2

Figure 3

A schematic representation of the relationship between human viruses and viruses from other mammals. Human viruses are depicted as a subset of mammal viruses, only partially protected by a species barrier. There are frequent minor incursions of zoonotic viruses (small arrows), and many of these may not persist in human populations. Occasionally there may be a much more significant event (large arrow) whereby a mammal virus proves capable of einvolving adaptation to infect and transmit from humans. doi:10.1128/microbiolspec.OH-0001-2012.f3