So far, the biggest outbreak has taken place on Vancouver Island in British Columbia, but the fungus has since spread past the order into Oregon where it's become a "a major source of illness in the region," according to the online journal PLoS Pathogens.
So far, five of 21 people contracting the fungus in the U.S. have died.
"This novel fungus is worrisome because it appears to be a threat to otherwise healthy people," said Edmond Byrnes of Duke University in North Carolina, who led the study.
The fungus, known as Cryptococcus gattii, was previously found in the tropics, where it has frequently been associated with Eucalyptus trees. But C. gattii was found in Vancouver Island in 1999 and has since been linked to disease in humans and animals, both in Canada and the United States. It spreads through spores carried by wind or when people or animals thread infected ground where the fungus is located.
"The continued expansion of C. gattii in the United States is ongoing, and the diversity of hosts increasing," the study says. It notes that cases have been observed in urban and rural areas, and have occurred in a variety of mammals. Although rare, little is currently known about how or why certain humans and animals have become infected. The study's authors warn that the threat C. gattii poses to agricultural and domestic animals "is significant and thus the need for cooperation among health officials is critical."