The Health Protection Agency (HPA) is contacting Britons who may have visited the United States national park at the centre of an outbreak of a deadly virus that has killed two people.
Around 100 UK travellers have been identified as having stayed at Yosemite National Park between mid-June and the end of August.
The agency said it was not aware of any cases of Hantavirus Pulmonary Syndrome (HPS) - which is carried by rodents - in Britons who had been there on holiday.
But it is warning people who had stayed at the park and also providing health advice and information on the situation in the US.
A total of up to 10,000 people who stayed in the lodging cabins in the park's Curry Village may have been exposed to the potentially fatal disease.
The California Department of Public Health has confirmed that six people who visited the park have now contracted the virus, up from four suspected cases earlier in the week.
Signs of the illness begin with flu-like symptoms which can take six weeks to incubate before rapid acute respiratory and organ failure.
More than 36% of people who contract the rare illness will die from it.
A spokesman for the HPA said: "Local HPA Health Protection Units are currently contacting UK travellers who may have stayed at the park using information supplied by the Yosemite National Park and providing health advice and information on HPS and information on the ongoing situation in the US.
"If individuals are concerned they can contact their local Health Protection Unit (HPU) for further advice."
The rare but severe respiratory disease is spread by contact with infected rodents, primarily deer mice.
Most people become infected by breathing in small viral particles from rodent urine or droppings that have been stirred up into the air.
The virus causing HPS in the US cannot be passed from person to person.
"There is no specific treatment for Hantavirus, but early recognition and supportive care can improve the outcome of this severe disease," the spokesman added.
"The HPA is liaising with the Centers for Disease Control and Prevention (CDC) and the European Centre for Disease Control and Prevention (ECDC) in responding to the situation."
More information on Hantavirus can be found on the HPA website at www.hpa.org.uk/Topics/InfectiousDiseases/InfectionsAZ/Hantaviruses/BackgroundI