(NEXSTAR) — A fungus that poses “an urgent antimicrobial resistance threat,” according to the Centers for Disease Control and Prevention, has already been detected in more than half of U.S. states.

Candida auris, or C. auris, is so concerning because it is often resistant to anti-fungal drugs, making it hard to treat infections. It can also be hard to identify with standard lab tests, making it even more difficult to treat properly and early.

The fungus was first detected in the United States in 2016. The fungus “spread at an alarming rate” between 2020 and 2021 in health care facilities, the CDC wrote in a memo released Monday, and has continued to spread in 2022.

By the end of 2022, there were more than 8,000 instances of Candida auris identified across 28 states and Washington, D.C. The map below shows confirmed cases of the fungus, with states shaded in red having the most infections.

While it usually isn’t a significant threat to healthy people, the CDC said, Candida auris outbreaks are a big threat in health care settings, where the fungus can enter patients’ bloodstreams and cause severe, deadly infections.

Sick people, those staying in the hospital long-term, and patients with invasive medical devices (like catheters or IVs) are most vulnerable.

The strain put on the health care system by the COVID-19 pandemic may have allowed the fungus to spread more quickly.

“The rapid rise and geographic spread of cases is concerning and emphasizes the need for continued surveillance, expanded lab capacity, quicker diagnostic tests, and adherence to proven infection prevention and control,” CDC epidemiologist Dr. Meghan Lyman said in a press release.

See the chart below for the number of clinical cases of Candida auris by state, according to CDC data:

While Iowa had zero clinical cases of Candida auris in 2022, it still had some “screening cases,” the CDC explained. That means the fungus was detected, but wasn’t known to be actively causing infection.