UK health officials are contacting around 100 Britons they believe could have been exposed to a deadly rodent virus while on holiday in the US this summer.
Six visitors to California's Yosemite National Park have so far been infected with Hantavirus Pulmonary Syndrome (HPS), a disease caused by breathing in infected rodent dropping or urine particles.
Hantavirus kills around 38% of people it infects. Of the six confirmed cases, two men have died.
It is not certain that any of those affected have been British citizens.
In a statement, the UK’s Health Protection Authority (HPA) said: "Hantavirus Pulmonary Syndrome is a rare but severe respiratory disease, which is sometimes fatal.
"It is spread by contact with infected rodents, primarily deer mice.
"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 recent cases have all been associated with people staying in Yosemite National Park's Signature Tent Cabins in the Boystown area of Curry Village.
Five of the six reported cases are known to have stayed in these cabins.
The HPA is only contacting British citizens known to have done the same, between the identified risk period from June 10 to August 24, 2012.
America's Centre for Disease Control estimates that around 10,000 travellers from all over the world stayed in the cabins during that time.
Park officials, who last week shut down the popular camping ground, said a design flaw in the cabins allowed mice to get inside the insulated walls.
It is believed the rodents' faeces, saliva and urine dried out and mixed with dust that was inhaled by visitors staying in the confined spaces.
Eating contaminated food or being bitten by infected rodents can also cause infection.
Tracking the outbreak has been made more difficult by the long incubation period of hantavirus, which can be up to six weeks.
According to the HPA, victims display flu-like symptoms, including fever, chills, headache, muscle aches, fatigue and a cough.
Other symptoms can include nausea, vomiting and abdominal pain.
The disease can then progress very rapidly, resulting in severe difficulty in breathing as the lungs fill with fluid.
Since the virus was first identified in the USA in 1993 there have been 556 confirmed cases.