How many viruses infect humans? Have we found them all? If not, how many more are there?
We conclude that it is extremely probable that new human viruses will continue to be discovered in the immediate future; we are not yet close to the end of the virus discovery curve. …Current trends are consistent with a pool of at least 38 undiscovered species that will be reported at an average rate of at least approximately one per year to 2020. … The upper limit for N is finite but large; we cannot rule out hundreds of novel human viruses to be reported in the future. 1
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| The discovery curve for human virus species. Cumulative number of species reported to infect humans (black circles and line). Statistically significant upward breakpoints are shown (vertical lines). Best-fit curve (solid line) and lower and upper 95% posterior prediction intervals (dashed lines) for extrapolation to 2020. 1 |
The article is free full-text; it includes a list of nearly 200 human viruses discovered from 1901 (Yellow Fever) through 2005 (Human T-lymphotrophic virus 4).
- Mark E.J. Woolhouse, Richard Howey, Eleanor Gaunt, Liam Reilly, Margo Chase-Topping, Nick Savill (2008). Temporal trends in the discovery of human viruses Proceedings of the Royal Society B: Biological Sciences, 275 (1647), 2111-2115 DOI: 10.1098/rspb.2008.0294[↩][↩]
It’s not about discovering ‘new’ human viruses, it is understandiing how many viruses there are in animals, because that is the source of new human infections. Steve Morse has theorized about this; summarized in my blog post at http://bitly.com/egut3, ‘The Zoonotic Pool’.
There’s no doubt that the zoonotic pool is enormous and contains a vast number of uncharacterized viruses. But realistically — at least, based on history — the risk of zoonotic viruses isn’t evenly distributed among that pool. For example, there may be 50,000 species of vertebrates, but about half of those are fish, and I’m not aware of any zoonotic diseases of any significance with a fish source. Mammals (around 5000 species) are disproportionate risks, with birds and reptiles trailing well behind, and amphibians not much more risk than fish (again, based on history).
(Vincent, I know you know this, but just for completeness … ) We have little idea what makes a virus potentially zoonotic. I’ve mentioned a few cases of viruses that jump species (here, here, here, and here) and one of the points I wanted to make is that while in hindsight we can sometimes point to characteristics of the virus that gave it that capability, I don’t think we’re able to predict any new events with any success. Gamma-herpesviruses show signs of occasionally jumping across species, albeit very rarely; beta- and alpha-herpesviruses seem to have stuck closely to a single species throughout their evolutionary history; yet poxviruses, which are also highly stable large DNA viruses, are relatively happy to leap from one species to another (rabbitpox and buffalopox are vaccinia virus, which in turn is a virus with no known natural host).
As always, more research is needed …
I say look to insects. The viral breeding ground.