Q. For most of my life I've suffered from what I'm told is "irritable colon" and have been chronically underweight. Recently I took a pill called Periactin for a severely itchy rash. The rash cleared up nicely, and to my surprise, so did my intestinal symptoms. I gained 10 pounds and look and feel great. Is there an explanation for this? When I stop the Periactin will my present state of well-being continue, or must I go back to being skinny and "crampy"? A. Irritable colon, also called irritable bowel syndrome, is a chronic intestinal condition that produces abdominal cramps, bloating, gassiness, constipation and loose stools, sometimes with mucus. Its cause isn't exactly known, but it seems to stem from a combination of psychological factors (stress, anxiety, worry) and an abnormality in bowel function. People with irritable bowel syndrome have "sensitive" intestines that react more vigorously to mental and physical stimuli.

Dietary fiber seems to play an important role in irritable bowel syndrome. Low-fiber diets may aggravate the condition, whereas high-fiber diets seem to help.

To get to your specific question, there is a good explanation for your serendipitous experience. Periactin is a type of antihistamine, the kind of medicine contained in many cold preparations. It also has anticholinergic action, meaning that it blocks certain actions of the nervous system, exerting a calming influence on the intestinal tract. So anticholinergics are often prescribed for people with irritable bowel syndrome, spastic colon or "nervous stomach."

My guess is that your symptoms will return once you stop taking Periactin, and a brief trial off the medicine would soon tell for sure. But if this simple remedy has given you this much relief, I'd recommend talking to your doctor about trying it out for your irritable colon symptoms, even though it's not normally used for your condition. Q. I was told that once you have mononucleosis, you develop an immunity to the disease and it won't recur. Since mono is a type of herpes, is it possible that having had mono makes you immune to genital herpes? A. In a word, no.

Let me clarify the relationship between mono and herpes. You're right that the virus that causes infectious mononucleosis (the Epstein-Barr virus) is one of the five members of the herpes family affecting humans. The others are cytomegalovirus (which sometimes causes an infection in fetuses), varicella-zoster virus (which causes chicken pox and shingles), herpes simplex type I (which causes fever blisters on the lips) and herpes simplex type II (which causes genital herpes).

These viruses are linked by a classification system that has more to do with their microscopic structure than similarity in the diseases they cause. It's like saying both humans and cows are mammals -- the similarities go only so far.

Once you develop mono, you're generally immune from getting it again. But having had mono, or even a herpes infection like a cold sore, doesn't protect you from getting a genital herpes infection.

Q. The first four winters we lived here after moving from Minnesota, my wife was sick all the time. She blamed the constant sore throat, swollen glands and headache on the house. Nonsense, I thought, until we traded places this past winter. My wife went to work each day and enjoyed her healthiest winter in years. I, on the other hand, worked at home, nursing a perpetual head cold and sinus condition. In retrospect, our youngest daughter seems to have had fewer colds once she started school. Is our house haunted by some health hazard?

A. There's a good chance you're all allergic to something in the house, possibly a mold. Seasonal allergies roughly follow a pattern -- tree pollens in the spring, grasses in the summer, ragweed and other weeds in late summer, and molds all year except winter. Dander, dust and house mites (microscopic creatures that feed on dust) could be a problem in the winter, when people tend to spend more time indoors. Trouble spots may lie in a damp basement, moldy shower or a mold-contaminated humidifier. Molds also can grow in the soil of potted plants.

I'd recommend dust-avoidance measures and a dehumidifier if needed. If these don't work, discuss the problem with your family doctor or an allergist, who can test you to pinpoint your specific allergy. Antihistamine medications may help in the meantime.

If your doctor agrees, you might try some of these techniques to allergy-proof your home, starting with your bedrooms:

*Replace feather pillows with hypoallergic (allergy-reducing) polyester ones, or put airtight plastic covers on them.

*Use a bedspread during the day and remove it (and any dust it gathers) from the bed each night.

*Remove rugs from the bedroom or choose types that can be kept dust-free.

*Avoid contact with pets, and keep them out of the bedroom.

*Reduce the amount of curtains, drapes and other dust-gathering items, particularly in the bedroom.

*Wash bedding and bed clothes in warm to hot water, which kills house mites.