Parasitic worms, bacteria and viruses are a constant feature of the daily lives of most 'healthy' populations of animal and plant species. My research is concerned with the population ecology of infectious diseases and the conservation of endangered and threatened species. Over the last eight years I have studied infectious diseases in a variety of endangered and fragile ecosystems. Each study has allowed me to develop sections of a larger body of theory that deals with the role of infectious diseases in wild animal populations. The role that infectious diseases play in driving populations to extinction is one of the key unsolved problems of conservation biology. Although ecologists now realize that pathogens and parasites play a key role in regulating population numbers, infectious diseases often cause rapid declines in the abundance of threatened species and continue to plague captive breeding programs. In particular, I have been studying rinderpest in Ngorongoro crater and brucellosis in the Yellowstone National Park, these pathogens infect both wild and domestic species and frequently cause problems around the edge of nature reserves where their control is traditionally undertaken by culling the wild hosts. This creates a classic conflict of interest between farmers who wish to eradicate the pathogen, and conservation biologists, who wish to conserve wildlife.