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.