anywhere in India and they have the right of way. Travel anywhere in the eastern United States and you’ll see squirrels, more than likely, as roadkill. Yet both species serve a similar epidemiological function: they receive bites from infected vectors that might otherwise have bitten humans, and they break the chain of pathogen transmission. In the case of Indian cattle, the bites are from mosquitoes infected with malaria parasites; the squirrels, on the other hand, receive their bites from ticks infected with the spirochetes that cause Lyme disease. In both cases, the presence of a relatively inefficient host species has reduced the rate of infectious disease spread into the human host population. Recently, ecologists have uncovered several other ways in which species diversity can benefit human health. In this Essay, we describe how disease risk is influenced by biological diversity and, specifically, how some host species act to reduce the risk of transmission of virulent zoonotic pathogens to people. This represents an exciting area of study where ecologists, conservation planners, and physicians can work together to reduce disease risk and maintain biological diversity. In a world where climate change may allow vectortransmitted diseases to spread from the tropics into the temperate zone, it may be sensible to conserve biological diversity for the purely selfish reasons of protecting human health. Zooprophylaxis One of the oldest examples of biological diversity reducing disease risk occurs with malaria and domestic livestock in India, and it may partly The Essay section contains opinion pieces on topics of broad interest to a general medical audience.