Cases of a deadly gut microbe that causes colon damage and severe diarrhea have been steadily rising in the U.S. during the past decade, health officials said this week. Called Clostridium difficile, aka either C. difficile or C. diff, the dangerous bug is commonly found in U.S. hospitals and kills tens of thousands of people a year, according to Centers for Disease Control and Prevention data published in the New England Journal of Medicine Thursday.

Health officials said the data highlight the growing threat of C. difficile. “This is very severe illness that causes tremendous suffering and death,” Michael Bell, deputy director of the CDC’s Division of Healthcare Quality Promotion at the National Center for Emerging and Zoonotic Infectious Diseases, told reporters Wednesday, according to NPR. The bacterium annually infects almost 500,000 Americans and leads to about 29,000 deaths, the health agency found.

Researchers estimate about two-thirds of C. difficile infections were picked up by patients during hospital stays. Another 100,000 were contracted by nursing-home residents. The rest occurred outside hospital settings and may have been related to visits to doctors’ or dentists’ offices, the CDC said.

“The numbers are pretty striking,” Anthony S. Fauci, director of the National Institute of Allergy and Infectious Diseases, told USA Today. “A substantial number of cases are occurring in people who have never been to a hospital.”

C. difficile infections are frequently treated with antibiotics. Doctors are often forced to remove a patient’s colon because of irreversible damage caused by the bacteria’s toxins. Health experts identified a new strain of the bacteria that emerged in 2000 and is transmitted more easily than other strains, according to NBC News.

Experts pointed to the overprescription of antibiotics in the U.S. as leading to more C. difficile infections. Antibiotics kill both the bad and the good bacteria in a patient’s body. Good bacteria keep bad bacteria in check. When antibiotics disrupt this balance in the human body’s natural microbiome -- the term for all microscopic life contained within the body -- it allows bacteria such as C. difficile to thrive. “If we can improve antibiotic prescribing, we can expect to see better C. diff rates,” said Bell. “Antibiotics are clearly driving this whole epidemic.”