When you take an antibiotic, it enters your bloodstream and travels through your body, killing bacteria but not human cells. There are few differences, however, between harmful and friendly bacteria. Antibiotics kill not only the bad bacteria making you sick, but also your resident friendly bacteria.

Friendly bacteria help keep you healthy in many ways, so when antibiotics kill friendly bacteria, your health can suffer because you lose these benefits. Additionally, losing friendly bacteria can give other types of bacteria room to multiply, leading to opportunistic infection. Sometimes opportunistic infection happens when bacteria from the environment get into your body and overrun friendly bacteria damaged by an antibiotic. Other times opportunistic infection begins when antibiotics disturb the balance of your resident microbes, and normally friendly bacteria multiply too quickly and become harmful.

One common cause of opportunistic infection is clostridium difficile (klos-TRID-e-uhm dif-uh-SEEL). Like many opportunistic bacteria, C. difficile live in the environment and do not normally harm healthy people. However, certain groups of people, like older adults who have been on antibiotics for a long time, are vulnerable. When antibiotics kill too many friendly bacteria in the intestine, C. difficile multiplies and produces toxins that make the person sick with symptoms like fever, nausea, diarrhea, and inflammation.