A deadly fungus that is considered an urgent public health threat by the Centers for Disease Control and Prevention spread at an “alarming rate” during the pandemic, the C.D.C. said on Monday.

The fungus, called Candida auris, preys primarily on older people with weakened immune systems and is particularly dangerous because it resists treatment by common antifungal medications. C. auris was first reported in the United States in 2016, showing up most notably in New York and Illinois, where public health officials hoped they could contain it by rigorous screening and infection control in long-term care facilities and nursing homes.

But over the course of 2021, state and local health departments around the country reported 1,474 clinical cases, more than a 200 percent increase over 476 cases in 2019.

The surge represents a “dramatic increase” in case load and transmission of C. auris, according to a research paper published Monday in the Annals of Medicine and compiled by researchers at the C.D.C. The fungus is now in half the states, many with just a handful of cases, but with higher concentrations in California, Nevada, Texas and Florida.

The new paper did not include caseloads from 2022. However, a C.D.C. website that tracks the spread of the fungus shows that there were 2,377 infections reported last year, another sharp increase.

Dr. Meghan Lyman, a medical officer in the mycotic diseases branch of the C.D.C., said that the agency did not have a good sense of how many deaths to attribute to the fungus. The reason is that people who become infected are also dealing with multiple other health challenges, so C. auris can be both a cause of death or something that, along with other poor-health factors, hastens it.

It is likely that the coronavirus pandemic worsened the spread of C. auris, C.D.C. officials said. With attention focused on Covid-19, less emphasis was put on screening for C. auris. Also, the fungus tends to cling to nursing gowns, gloves and other personal protective gear that, under ideal conditions, would be changed frequently but that were reused during pandemic because of supply shortages. C. auris can also attach to ventilators or other medical equipment

“We were worried what would happen during Covid,” Dr. Lyman said. She characterized the spread as “concerning but not surprising.”

C. auris is not a particular threat to young healthy people, whose immune systems can fight it off, but can be transported on skin and clothing. The fungus commonly strikes older patients, particularly those who have many visits or prolonged visits to health care facilities, where it can be hard to clean or eradicate.

The challenge in treating C. auris stems from the fact that it can be resistant to antifungal medications. During 2020, the research paper found, 86 percent of the germ samples tested by the C.D.C.’s Antimicrobial Resistance Laboratory Network were resistant to a class of drugs known as Azoles.

More concerning to health officials is that 1.2 percent were resistant that year to a frontline treatment class of drugs called echinocandins. If resistance to echinocandins becomes more common as the germ evolves, C. auris could become extremely difficult, if not impossible, to treat, health officials said.

Dr. Lyman also said that the news is not all bad. Intensive efforts to stop the spread of the germ in New York and Illinois appear to have been effective in containing C. auris within the health care systems in those states — even as the bug rooted elsewhere.

“It’s not a hopeless situation,” Dr. Lyman said.