Some emerging infections are caused by microbes that originate in nonhuman vertebrates. Hantavirus pulmonary syndrome was first noted in New Mexico in 1993. The disease is caused by Sin Nombre virus, which is endemic in the deer mouse namely Peromyscus maniculatus. The virus is excreted in mouse droppings, and contaminated blankets or dust from floors provided opportunities for human infection. Because humans are not the natural host for Sin Nombre virus, the human disease is rare. The emergence of Lyme disease followed a similar course. The infection has probably been present in North America for millennia, but it was only in the 20th century that conditions in the northeastern US changed to favor the propagation of Lyme disease. Two other viruses that probably originated in nonhuman vertebrate hosts are HIV and the SARS coronavirus. HIV, the cause of AIDS, most likely arose from interspecies transmission between nonhuman primates and humans. Because of these public health is naturally affected.