Antibiotic associated diarrhea is caused by

Inflammation of the colon caused by the bacteria Clostridium difficile. Clostridium difficile colitis results from disruption of normal, healthy bacteria in the colon, often as a result of antibiotics. C. difficile can also be transmitted from person to person by spores. It can cause severe damage to the colon and even be fatal Antibiotic-associated diarrhoea is among the most common adverse events related to antibiotic use. Most cases are mild, but Clostridium difficile infection causes a spectrum of disease, ranging from occasional diarrhoea to colitis, toxic megacolon, and potentially death Ref Harrison20th edition pg 1123