DEAR DR. DONOHUE: A little more than a month ago, I was hospitalized for pneumonia. I spent a full week in the hospital, taking antibiotics. I was still taking them when I went home. That’s when I developed diarrhea. My doctor told me to stop the antibiotics, and I did. The diarrhea continued, and I had to be readmitted to the hospital. They put me on a different antibiotic. I think I’m OK now, but I wonder about all this. What exactly went wrong? — D.P.

ANSWER: You had antibiotic-associated colitis, the cause of which is a bacterium by the name of C. difficile. It happens like this: People take an antibiotic for an infection, like your pneumonia. The antibiotic is somewhat indiscriminate in killing off bacteria. In this instance, it kills off the good bacteria in the colon, the ones that keep the digestive tract in balance. Then the C. difficile bacterium, often found in the hospital environment, takes over the vacuum left by the death of the good bacteria. Soon it becomes the predominant bacterium. Any antibiotic is capable of spawning the proliferation of C. difficile, but at the top of the list are clindamycin, ampicillin, cephalosporins and fluoroquinolones.

C. difficile makes toxins (poisons) that produce diarrhea. In the United States, estimates indicate that there are as many as 750,000 infections with the bacterium every year. Invariably, the person is on or has been taking antibiotics.

Stopping the antibiotic puts an end to diarrhea in seven out of 10 cases. That still leaves a significant number of people with diarrhea so severe that it can be life-threatening. Metronidazole and vancomycin are two antibiotics that usually can eradicate C. difficile.

The Food and Drug Administration has approved a new drug for antibiotic-associated colitis. It’s fidaxomicin (Dificid). It gives doctors additional ammunition to treat this illness should other remedies fail.

DEAR DR. DONOHUE: What can you tell me about osteomyelitis? I have it. It started out as severe pain in my lower back. I thought I had a back strain. Three months and four doctors later, I found out that one of my backbones had an infection. How long does this have to be treated? I’ve been taking an antibiotic for three weeks. My back has stopped hurting. How did I get this infection? — L.H.

ANSWER: Bone infection, osteomyelitis, often is difficult to diagnose and difficult to treat. Germs get to bones in a number of ways. One is spread through the blood from an infection in another part of the body — a boil, for example. Or an infection adjacent to a bone can seed the bone with germs. A broken bone that penetrates through the skin is another way for bone infection to start.

The length of time to treat a bone infection must be individualized. Infection of backbones often calls for four to six weeks of treatment. Sometimes the duration of therapy has to be much longer. The fact that your pain is gone is a sign that you’re winning the battle.

DEAR DR. DONOHUE: Every day of our married life, my husband drank in excess. He died last year.

Before his death, he made no sense. I thought he had come down with Alzheimer’s disease, but his doctor said he had hepatic encephalopathy. I wasn’t clear about what this is, but I didn’t pursue it then. What is it? — J.K.

ANSWER: It’s brain shutdown due to liver failure. The liver isn’t able to detoxify cellular-waste products. One of those products is ammonia, which is poison to the brain.

In the early stages, the sign of encephalopathy is confusion. If medicines can’t right the problem, confusion is followed by agitation, then coma and death.

Dr. Donohue regrets that he is unable to answer individual letters, but he will incorporate them in his column whenever possible. Readers may write him or request an order form of available health newsletters at P.O. Box 536475, Orlando, FL 32853-6475. Readers may also order health newsletters from www.rbmamall.com.

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