Human viruses: discovery and emergence

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

From previous reviews of the literature [25, 26, 34] , it is possible to put approximate numbers of virus species at each level of the pyramid. We know that there are greater than 200 viruses at least at level 2 (see §2a). We do not have a good estimate of the total species diversity of mammalian and avian viruses; however, we can get an indirect indication of the magnitude of the barrier between level 1 and level 2. It has been reported elsewhere (R. Critchlow 2010, personal communication) that of the virus species known to infect domestic animals (livestock and companion animals)-to which humans are presumably routinely exposed-roughly one-third are also capable of infecting humans. The species barrier exists: but it is clearly very leaky. Based on data from [25] , roughly 50 per cent of the viruses that can infect humans can also be transmitted by humans (level 3), and roughly 50 per cent of those are sufficiently transmissible that R 0 may exceed one (level 4). That a significant minority of (mammalian or avian) viruses should be capable of extensive spread within human populations (or of rapidly becoming so [35] ) is consistent with experience: there are several examples within the past hundred years alone (HIV-1, SARS, plus variants of influenza A) and many more in the past few millennia (e.g. measles, mumps, rubella, smallpox). It is noteworthy that the 'shape' of the pathogen pyramid for viruses is very different to that for other kinds of pathogen (bacteria, fungi, protozoa or helminths), of which much smaller fractions are capable of extensive spread in human populations (data from [25] ). The most straightforward explanation for this is the much more rapid evolution of viruses (especially RNA viruses), allowing them to adapt to a new (human) host much more quickly than other kinds of pathogen.