Up to 10,000 people who have stayed at Yosemite Park between June and August have been urged to be vigilant after two people died from Hantavirus.
The US Center for Disease Control and prevention has urged that lab testing be conducted on people who have symptoms consistent with lung disease Hantavirus Pulmonary Syndrome.
The virus starts out causing flu-like symptoms, including headache, fever, muscle ache, shortness of breath and cough, and can lead to severe breathing difficulties and death.
The incubation period for the virus is typically two to four weeks after exposure, with a range between a few days and six weeks. Just over a third of cases are fatal.
Two men have died from hantavirus linked to the Yosemite outbreak and four others were ill.
Most of the victims were believed to have been infected while staying in one of 91 tent-style cabins in Yosemite's popular Curry Village camping area.
Yosemite officials shut down all 91 of the insulated tent cabins after finding deer mice, which carry the disease and can burrow through holes the size of pencil rubbers, nesting between the double walls.
Nearly 4m people visit Yosemite, one of the US's most popular national parks, each year.
Roughly 70% of those visitors congregate in Yosemite Valley, where Curry Village is located.
