In this paper, we take an ecological approach to studying the diversity of human viruses (defined as viruses for which there is evidence of natural infection of humans). These new viruses differ wildly in their importance, ranging from the rare and mild illness due to Menangle virus to the devastating public health impact of HIV-1. New species of human virus are still being identified, at a rate of three or four per year (see below), and viruses make up over two-thirds of all new human pathogens, a highly significant over-representation given that most human pathogen species are bacteria, fungi or helminths. For this reason, an effective global surveillance system for novel viruses is needed.ДЇollowing on from the discovery of tobacco mosaic virus in 1892 and foot-and-mouth disease virus in 1898, the first вЂfilterable agentвЂ™ to be discovered in humans was yellow fever virus in 1901. It seems almost inevitable that new human viruses will continue to emerge, mainly from other mammals and birds, for the foreseeable future. A few possible predictors of species jumps can be identified, including the use of phylogenetically conserved cell receptors. 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. Many specialist human viruses also have mammalian or avian origins. More than two-thirds of human viruses can also infect non-human hosts, mainly mammals, and sometimes birds. 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. 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. There are 219 virus species that are known to be able to infect humans.

Google worm virus