Candida auris, a potentially deadly fungus that is resistant to drugs, is spreading quickly in U.S. health care facilities, the Centers for Disease Control and Prevention (CDC) warned this week. According to federal data, clinical cases of the fungus almost doubled in 2021.

The CDC noted that the disease primarily affects older individuals and those with weakened immune systems, especially those who have been hospitalized for an extended period. Candida auris may not always cause infection and can be carried on a patient’s skin, making it easier to spread to others. However, when it does cause an infection, the agency said, it may be difficult to identify because it most commonly occurs in patients who are already ill with other diseases and showing symptoms.

The CDC stated that the most common symptoms of the fungus are fever and chills that do not improve after antibiotics.

Based on CDC data from a limited number of patients, 30-60% of individuals with Candida auris infections have died. However, “many of these people had other serious illnesses that also increased their risk of death,” the agency added.

Individuals who are young and healthy and whose immune systems can fight off the fungus are usually not at risk. The CDC is notifying U.S. health care facilities to be aware of Candida auris in their patients.

Reported cases of Candida auris have increased from 53 in 2016 to 1,474 cases in 2021. There were 2,377 confirmed cases last year. The fungus has reportedly been found in half of all U.S. states, and health officials believe the pandemic has likely exacerbated its spread.

Candida auris was first reported in Japan in 2009, but researchers have tracked the earliest strain to South Korea in 1996.