Clostridioides difficile (klos-TRID-e-oi-deez dif-uh-SEEL) is a bacterium that causes an infection of the colon, the longest part of the large intestine. Symptoms can range from diarrhea to life-threatening damage to the colon. The bacterium is often called C. difficile or C. diff. Illness from C. difficile often occurs after using antibiotic medicines. It mostly affects older adults in hospitals or in long-term care settings. People not in care settings or hospitals also can get C. difficile infection. Some strains of the bacterium that can cause serious infections are more likely to affect younger people. The bacterium used to be called Clostridium (klos-TRID-e-um) difficile.