The U.S. Centers for Disease Control and Prevention (CDC) this week warned that carbapenem-resistant Enterobacteriaceae (CRE) are becoming more common. The bacteria can cause potentially deadly infections, but are also highly resistant to antibiotics, making the infections difficult to treat.

Though the germs are a normal part of the human digestive system, some of them have evolved the ability to resist antibiotics. If these resistant CRE enter the blood or other areas of the body they can cause severe infections that kill up to half of patients who become infected. The bacteria can also pass their antibiotic-resistance to other germs, making other types of infections hard to treat.

The CDC’s latest Vital Signs report shows that CRE are becoming more common in medical facilities, and one specific type has been found in 42 states. Though healthy people aren’t normally infected with CRE, patients on ventilators, patients with catheters, or those on long courses of antibiotics are at risk. Though The infections are still relatively uncommon, but CDC is calling on medical facilities and doctors to improve the way they prescribe antibiotics.

Just last month a new study found that patients with CRE could take over one year to be rid of the bacteria, even after their infections have ended. This raises the possibility that patients positive for CRE could become re-infected or infect others.