Mediaboss Marketing

13 September 2012 Last updated at 20:12 ET

Yosemite park rangers have expanded a health advisory about a killer virus to nearly a quarter of a million people who have stayed there since June.

The mouse-borne hantavirus is now known to have infected nine people - three of whom have died. There is no known cure.

Officials at the California park insist their letters to more than 230,000 people are purely precautionary.

Deer mice, which spread the infection, have increased in number and now the rodents are being trapped and killed.

Yosemite initially sent a warning last month to 1,700 campers who stayed at a specific campsite, before gradually expanding that figure as infections emerged elsewhere.

Most of the cases involved guests at the Signature cabins in Curry Village, but one case involved someone who had stayed at multiple High Sierra camps in wilderness areas.

A ninth person is now recovering after being stricken with the disease following a visit to Yosemite in early July, said National Park Service spokesman John Quinley on Thursday.

It can take six weeks for the emergence of early symptoms such as aches and fever, or in half the cases headaches and sickness. It then moves into the lungs.

One third of cases are fatal, but identifying it early through blood tests increases a person's chance of survival.

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Yosemite virus alert to 230,000