Humans have lived in close proximity to other animals for thousands of years. Recent scientific studies have even shown that the presence of animals has a positive effect on our physical and mental health. People with pets typically have lower blood pressure, show fewer symptoms of depression, and tend to get more exercise.

But there is a darker side to the relationship between animals and humans. Animals are carriers of harmful infectious agents and the source of a myriad of human diseases. In recent years, the emergence of high-profile illnesses such as AIDS, SARS, West Nile virus,...