Although foot-and-mouth disease (FMD) virus does not infect humans, veterinarians consider it the most dangerous livestock pathogen that could be introduced into the United States, either inadvertently or deliberately. The disease is caused by a virus that affects only ruminant animals with cloven hooves, including cattle, swine, sheep, and goats, as well as more than 70 species of wild animals, such as deer and wild boar.