They have confirmed that a ninth person has been diagnosed with hantavirus, a rodent-borne illness blamed for three deaths connected to the park in California.
National Park Service spokesman John Quinley said the victim, from California, had stayed in the ‘Signature’ cabins in the park’s historic Curry Village in early July.
The majority of the cases involved guests at the cabins. One visitor stayed at High Sierra camps in Yosemite’s wilderness areas.
The disease – for which there is no known cure – is carried in the faeces, urine and saliva of deer, mice and other rodents, and carried on airborne particles and dust.
People can be infected by inhaling the virus or by handling infected rodents.
Infected people usually have flu-like symptoms including fever, shortness of breath, chills and muscle and body aches.
The disease – which is spread by mice – can take up to six weeks to incubate.
The officials say their warning, which has gone out to more than 230,000, is precautionary.