Human viruses: discovery and emergence

Woolhouse, Mark; Scott, Fiona; Hudson, Zoe; Howey, Richard; Chase-Topping, Margo

The lines of evidence described earlier combine to suggest the following tentative model of the emergence process for novel human viruses. First, humans are constantly exposed to a huge diversity of viruses, though those of others mammals (and perhaps birds) are of greatest importance. Moreover, these viruses are very genetically diverse and new genotypes, strains and species evolve rapidly (over periods of years or decades). A fraction of these viruses (both existing and newly evolved) are capable of infecting humans. It is not clear whether some of these human-infective viruses will already be capable of reaching higher levels of the pathogen pyramid-so-called 'offthe-shelf ' pathogens-or whether subsequent evolution of their ability to infect and transmit from humans is usually required-'tailor-made' [31] . The distinction is potentially important as it implies different determinants of the rate of emergence of viruses with epidemic or pandemic potential: for off-the-shelf pathogens this rate is largely driven by the rate of human contact with a diversity of virus genotypes (possibly rare genotypes) within the non-human reservoir (i.e. ecology); for tailor-made viruses, the key variable is likely to be the rate of genetic adaptation within the new human host (i.e. evolution) [35] .