A standard method for estimating numbers of species is to extrapolate the cumulative species discovery curve (Bebber et al. 2007) . We gathered data for this curve by systematically searching the primary literature for first reports of human infection with each of the currently recognized virus species, using species as defined by the International Committee on Taxonomy of Viruses (ICTV; http://www.ictvonline.org/). We note that the set of viruses we are interested in-those that can infect humans-is a small subset of the total (over 1500 species according to ICTV ) and, as is discussed below, not a closed set because many of these viruses can also infect other hosts . We regard this as analogous to constructing species discovery curves for any subdivision of geographical range or habitat. As we demonstrate below, this approach yields an excellent description of the discovery curve.