The list of infectious agents that are part of the causal chain of cancer continues to grow. Persons infected with H. pylori have a higher rate of stomach cancer than people not infected. Human papilloma viruses are responsible for a large number of cervical cancer cases. Several animal cancers have infectious causes, and there is good reason to believe that viruses and bacteria will be found to cause additional types of human cancer as well. The presentations at the International Conference on emerging Infectious Diseases 2000 made clear that most of the emerging infections of humans are zoonoses.
Humans have increasing contact with both domestic and wild animals.
Suburban and rural developments encroach into animal habitats, and previously isolated regions around the world are being settled. Add to this the ability to travel from just about any place on earth to another in less than 24 hours, and the opportunity for cross-species transmission grows. Even HIV was probably acquired by humans from animals. The zoonotic disease with perhaps the greatest potential for damage worldwide is influenza. In 1918, an influenza virus that is thought to have spread from swine to humans killed millions of people.