LOS ANGELES — A potentially lethal fungus normally found only in the tropics has established a foothold on the Pacific Coast of British Columbia and has slowly made its way southward into Washington, Oregon and Idaho, researchers said Thursday.

Health authorities are not unduly alarmed by the fungus because the number of cases so far remains small, but both federal and state officials are monitoring its progress in the fear that it will spread more rapidly as it reaches warmer climates.

The spread also is a concern because the strain of the fungus that moved into the United States in 2004 has mutated to become more lethal than the original strain that invaded British Columbia in 1999.

Five of the 21 people who contracted the fungus in the United States have died (about 25 percent), compared to 8.7 percent of the 218 infected people in Canada. The fungus also has infected many different species of mammals.

The fungus in question is called Cryptococcus gatii, which is found primarily in tropical and subtropical regions.

C. gatii seems to infect primarily healthy people, while its close relatives — for which it can easily be mistaken — primarily infect those with a compromised immune system.

It is not transmitted from person to person or carried by insect or animal vectors. C. gatii, like all fungi, forms spores that are blown in the wind or moved around by disturbances of the soil. They can easily be carried on the soles of shoes, for example.

The spores are inhaled and colonize the lungs, then they spread throughout the body. The primary symptoms include prolonged coughing, night sweats, pneumonia and weight loss over a period of weeks. Meningitis can occur, which produces severe headaches. Treatment involves six to eight weeks of intravenous antifungal medications, such as amphotericin B, followed by up to six months of oral fluconazole.

"We need physicians to be aware of this and think about it when they see symptoms of infection," said epidemiologist Julie Harris of the U.S. Centers for Disease Control and Prevention. "This is a serious infection, but so far it's a rare infection."