In the simplest terms, we all have bacteria that live in the large intestine, or colon. Some of those bacteria are "good", in that they provide a function to colon health, and some are bad. Sometimes, particularly in ill, hospitalized or nursing home patients, good bacteria can be killed off and bad bacteria can overtake them, usually this is from antibiotics. When that occurs, the colon becomes inflamed, creating a condition known as colitis. That process can become so severe that the patient develops sepsis, or an overwhelming systemic response to infection, and in those cases removal of the entire colon is the only treatment to save the patient's life.
Blood clots, particularly those that form in the veins of the legs, are a severe risk with sepsis and surgery. As providers, we take measures to reduce those risks, but sometimes they still occur. Sometimes those clots are some extensive that the only choice we have is to amputate a limb, although that is in extreme cases.
I hope this helps answer your questions.