Our analysis is based on the rate of accumulation of new human virus species: the 'discovery curve'. Discovery curves have previously been used to estimate the total diversity of various plant and animal taxa (Dove & Cribb 2006; Bebber et al. 2007) . However, to our knowledge, the discovery curves have not previously been compiled for any category of human pathogen. Having compiled the discovery curve, we proceed to develop a simple statistical model which we use to estimate the size of the pool of human virus species, N, and the expected rate of discovery of new species to 2020.