A new strain of a virulent, deadly fungus has spread through the Pacific Northwest as climatic changes have helped the organism thrive in temperate regions it wasn't comfortable in earlier.

The Centers for Disease Control and Prevention issued its first advisory asking doctors to watch out for Cryptococcus gattii infections last week in a Morbidity and Mortality Weekly Report. The fungus typically inhabits tropical and subtropical nations such as Australia and New Zealand, and is found growing passively on trees.

Since 2004, 60 infections have been found in the United States -- not a significant cause for concern, say researchers -- but the disease can be fatal when acquired, according to the CDC. The most recent infections have been found in California, Oregon and Washington state.

People and animals inhale the spores, which cause Cryptococcosis, a yeast infection that manifests as pneumonia or meningitis. It cannot be prevented or easily treated.

"More than half of the patients have no underlying risk factors, which is worrying in context of overall health burden and disease," said Dr. Joseph Heitman, a professor in the department of molecular genetics and microbiology at Duke University Medical Center.

The reason for the greater spread of the emerging infectious disease is unknown. It was first found in North America in 1999, on Vancouver Island off the coast of British Columbia, and has since resulted in about 220 cases.

"The organism wasn't supposed to be here," said Karen Bartlett, an associate professor at the School of Environmental Health at the University of British Columbia, who has been tracking the fungus since it first emerged. "In all literature, in all texts, it is an organism associated with the environment. It is not a cause for concern, except for the fact that it can be pathogenic for mammals."

A related fungus, Cryptococcus neoformans, infects patients with HIV who have weakened immune systems and causes pneumonia and meningitis.

The two fungi are distinct from each other, having diverged in evolution nearly 40 million years ago, according to Heitman. Until 2001, however, doctors were diagnosing all Cryptococcus infections as having emerged from neoformans. It was only when relatively healthy people started exhibiting symptoms that it became apparent that a second fungus was responsible.

Patients infected by C. gattii tend to have more lesions in the brain and the lungs, with larger spaces filled with bacteria that can be difficult to treat, according to Heitman.

Three different strains of C. gattii have been found in the United States, with researchers at Duke University finding a strain unique to the forests of Oregon that appears to be hypervirulent. Their work was published in the journal Public Library of Science Pathogens in April. The yeast, which finds a home within cells of the immune system called macrophages, was able to reproduce very successfully within its host.

How the fungus got to the Pacific Northwest, leaving behind a trail of animal and human disease, remains a mystery. But Bartlett thinks that a changing climate with less severe winters definitely helped it thrive in its new habitat. The organism likes heat and sunlight.

"Give it a combination of what it likes, and don't give it very cold winters," said Bartlett, referring to rising temperatures in British Columbia. Over the past 40 years, temperatures on Vancouver Island have risen by 0.5 to 1 degree Celsius, said Bartlett. It is likely to colonize the Puget Sound area, where land, climate and vegetation are similar, she said.

"Given our current inability to control greenhouse gases, temperature isn't going to go down soon, so the fungus is not going to go away," said Bartlett.