Human viruses: discovery and emergence

Abstract

There are 219 virus species that are known to be able to infect humans. The first of these to be discovered was yellow fever virus in 1901, and three to four new species are still being found every year. Extrapolation of the discovery curve suggests that there is still a substantial pool of undiscovered human virus species, although an apparent slow-down in the rate of discovery of species from different families may indicate bounds to the potential range of diversity. More than two-thirds of human viruses can also infect non-human hosts, mainly mammals, and sometimes birds. Many specialist human viruses also have mammalian or avian origins. Indeed, a substantial proportion of mammalian viruses may be capable of crossing the species barrier into humans, although only around half of these are capable of being transmitted by humans and around half again of transmitting well enough to cause major outbreaks. A few possible predictors of species jumps can be identified, including the use of phylogenetically conserved cell receptors. It seems almost inevitable that new human viruses will continue to emerge, mainly from other mammals and birds, for the foreseeable future. For this reason, an effective global surveillance system for novel viruses is needed.

Figure 1.

Discovery curves for human viruses. (a) Virus discovery curve by species. Cumulative number of species reported to infect humans. Statistically significant upward breakpoints are shown (vertical lines). (b) Virus discovery curve by family. Cumulative number of families containing species reported to infect humans.

Figure 2.

Patterns in human virus diversity. (a) A pie chart showing the continent where human virus species were first reported (n = 215, with four species not assigned to a continent). (b) Species abundance histogram for human viruses by family. Twenty three families are represented; six virus species remain unassigned to a family.

Figure 3.

The pathogen pyramid (adapted from [30]). Each level represents a different degree of interaction between pathogens and humans, ranging from exposure through to epidemic spread. Some pathogens are able to progress from one level to the next (arrows); others are prevented from doing so by biological or ecological barriers (bars)—see main text.

Figure 4.

Number of virus species with broad (blue bars) or narrow (red bars) host range as a function of the percent homology of the cell receptor used (see main text).