Infection rates from drug-resistant “nightmare bacteria” rose almost 70% between 2019 and 2023, according to a new report from Centers for Disease Control and Prevention scientists.

Bacteria that are difficult to treat due to the so-called NDM gene primarily drove the increase, CDC researchers wrote in an article published Monday in the Annals of Internal Medicine. Only two antibiotics work against those infections, and the drugs are expensive and must be administered through an IV, researchers said.

Bacteria with the gene were once considered exotic, linked to a small number of patients who received medical care overseas. Though the numbers are still small, the rate of U.S. cases jumped more than fivefold in recent years, the researchers reported.