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

New human viruses
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).

  1. 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[][]