Six visitors to California's famous Yosemite National Park have now been infected with a rare rodent-born virus, two of whom have died, officials said Thursday, in an update on the outbreak.

Earlier this week Yosemite authorities closed down all tent cabins in part of Curry Village, a popular lodging area in Yosemite Valley, the tourist hub at the center of the scenic park visited by millions of people every year.

The National Park Service (NPS) has written to some 2,900 parties who stayed in the Boystown area tent lodgings between June 10 and August 24, alerting them to keep an eye out for symptoms of hantavirus pulmonary syndrome (HPS).

The disease can take up to six weeks to incubate after exposure to the virus, usually through contact with the urine, droppings or saliva of infected rodents, primarily deer mice.

The California Department of Public Health (CDPH) reported four more cases Thursday, for a total of six. A Pennsylvanian and a Californian have died, three others have recovered and one is currently hospitalized but improving.

CDPH head Ron Chapman said his department and the NPS are working with the Centers for Disease Control and Prevention to probe the Yosemite outbreak "and reduce the risk of other visitors becoming ill from this virus.

"CDPH is continuing to monitor cases of Hantavirus Pulmonary Syndrome in persons who visited Yosemite National Park," he added.

Hantavirus is a rare but serious disease and early medical attention is "critical" to limit its effect. It begins with fever and aches, but can progress rapidly to a life-threatening illness, the NPS said.

Since the disease was first identified in 1993 there have been some 60 cases in California and 587 cases nationwide in the United States, around a third of which have been fatal.