Arenaviruses cause the South American haemorrhagic fevers, which produce hundreds of cases annually, with a case-fatality ratio as high as 33%. Hantaviruses cause hundreds of thousands of cases of haemorrhagic fever with renal syndrome (HFRS) in Europe and Asia each year, but were thought to be rare or absent in the USA until the appearance of HFRS in 1993. Since 1993, at least 20 additional hantaviruses have been isolated from rodents throughout the Americas; about half are known human pathogens.
Although arenaviral diseases of humans (other than lymphocytic choriomeningitis associated with the introduced Old World rodent Mus musculus) have not been recognized in North America, Tamiami virus has been recognized in association with cotton rats Sigmodon hispidus since 1969, and Whitewater Arroyo virus was identified from wood rats Neotoma spp. in the southwestern United States in 1995. The potential of Whitewater Arroyo virus for causing human disease is under investigation.