Thirteen people in the United States have been sickened by a deadly form of drug-resistant yeast that has spread across the world since 2009, and four of those people have died, the Centers for Disease Control and Prevention said on November 4.

The new yeast superbug, called Candida auris, causes outbreaks in a way that has never been seen before, and it doesn’t respond to the few drugs available to treat fungal infections. CDC scientists are urgently studying it to understand how it spreads and how it can be controlled.

“I am worried,” says Tom Chiller, a physician who is chief of the CDC’s mycotic diseases branch. “I think this is a real threat.”

C. auris is causing very serious wound and