Hantaviruses are a large group of viruses, carried in rodents and insectivores worldwide, which can cause disease in people who become accidental hosts. Each virus appears to have co-evolved with its reservoir host, and does not usually cause illness in this animal. In humans, the consequences of infection depend with the virus. Although some hantaviruses tend to be associated with asymptomatic infections or mild disease, others have case fatality rates of 50% or greater. Hantavirus infections are fairly common in parts of Asia and Europe. Although hantavirus-associated disease was first reported in the United States in the 1990s during an outbreak in the Four Corners region, these viruses are not new to the U.S. Since the 1990s, they have been reported in rodents and insectivores throughout the country, and additional human cases have been found. There is serologic evidence that some domesticated animals may also become accidental hosts for hantaviruses, but little or no evidence of disease.