Noting that p and N are highly correlated (figure 3), our best estimate for p is 0.015 (95% credible interval, 0.004-0.026) with 117 (38-562) so far undiscovered virus species. Extrapolating the discovery curve, allowing for parameter uncertainty and stochastic discovery, we obtain a best estimate of 22 new species (10-40) by 2020 ( figure 1) .