DEAR DR. DONOHUE: My grandson, 14, came down with rheumatic fever last summer. He got a heart valve problem from it. He has to have a penicillin shot monthly until he is 25. Will he need a plastic valve? When? Could a common cold be fatal to him? - Mrs. L.A.

ANSWER: A few children with untreated strep throat develop rheumatic fever signs a few weeks later. Temperature rises, joints may swell and ache, and rashes may develop.Most important of all, heart symptoms may appear. This is worrisome, for while the rash and other symptoms resolve on their own, the heart disturbance may bode ill for the future. This usually entails damage to a heart valve, as in your grandson's case.

The damage is not from the strep throat germ itself, but from the protective antibodies it triggers. Those antibodies can be destructive if they zero in on the heart valve as a mistaken target.

The heart valve damage can't be undone, so if it is serious, replacement is in order. Minor damage can usually be tolerated without surgery. If not, the scheduling of surgery is dictated by careful monitoring of valve status.

The penicillin protects against a second strep infection and a potential repeat antibody attack on the weakened valve. Doctors are not of one mind as to how long penicillin protection is needed. Some say for life. Others advise use into the mid 20s if there has been no strep infection in the preceding five years. Viral infections such as a common cold do not pose the threat a second strep assault does.

With the care he's getting, your grandson's outlook is good. I gather from your letter that the heart valve damage was mild. Rheumatic fever, after a long hiatus, has reemerged recently as a major threat, calling for prompt diagnosis and treatment of strep throat.

DEAR DR. DONOHUE: Are bunions inherited somehow? My mother's feet and mine are disaster areas. But my sister's are perfect. - R.O.

ANSWER: Bunions form because of a peculiar foot structure that places the large toe bone out of alignment. This inherited problem is aggravated by improper footwear, which confines the toe. Your sister apparently missed your genetic fate. However, anyone with the family trait ought to pay close attention to shoe selection, permitting plenty of space for the misaligned toe joint. See the material on foot care. Write to Dr. Donohue/No.11, Box 830, Gibbstown, NJ 08027-9909, enclosing a long, stamped, self-addressed envelope and $2.

DEAR DR. DONOHUE: My wife hurt her ankle in aerobics dance class. She had to drop out. I think the dangers for some women should be made clear. A word from you might help. - E.Y.

ANSWER: Most aerobics dance injuries occur in the lower leg from overuse. The second important contributor is poor shoes, especially when there has been a previous foot injury. Inappropriate dance surfaces also contribute. Beware unpadded concrete surfaces. Soft cushioned shoes are a must.

Most aerobics dance participants withstand all the rigors unharmed. In fact, instructors are the usual victims (overuse).

FOR S.A. - One study indicates that on average, when a woman discontinues birth-control pill use she can expect on the order of six menstrual cycles of relative infertility. This hiatus can last up to a year, however. Women vary in the amount of time needed to counter the pill's hormonal effect of preventing ovulation.

DEAR DR. DONOHUE: During a routine examination, the doctor said that my platelet count was excessive. I had a bone marrow test, also chest X-rays and spleen and liver scans. All were normal. The doctor calls my condition "essential thrombocytosis." What is this? What are the causes? Is it a serious problem? - J.M.C.

ANSWER: Thrombocytes and platelets are the same things, blood constituents that form clots when blood vessels are broken.

Something is causing overproduction of thrombocytes, and your testing was to find out what that something is. Causes can involve bone marrow, where platelets are produced, or the spleen and liver, where those substances are removed. Your tests failed to turn up any clues to such problems. That's why your thrombocytosis was labeled "essential," in effect, cause unknown.

Yes, this can be a serious problem. Such thrombocytes may be defective and lead to either bleeding from or clotting within blood vessels.

There are ways to control conditions. But many times, treatment is not favored in cases like yours. Your bone marrow is OK, as is your liver and spleen. You apparently have no symptoms from the condition. In this setting, the feeling sometimes is that medicines commonly brought into play might create new problems. Therefore, it can be better to simply monitor the patient and not intervene until occasion demands.

DEAR DR. DONOHUE: A doctor has diagnosed my 30-year-old niece as having Wilson's disease. She has copper deposits in her liver. Could you please explain this disease? Thank you. - H.B.

ANSWER: Wilson's is an inherited illness in which the body loses the ability to handle and rid itself of copper, which then begins saturating organs, especially the liver and brain.

The treatment is to remove the copper before it causes organ deterioration. Drugs are used to accomplish this, chiefly one called penicillamine (unrelated to penicillin).

Wilson's isn't the easiest disease to diagnose, but once it is, then treatment should correct things. It is only when the problem goes undiagnosed that serious problems arise. Naturally, since it is an inherited condition, close relatives should be checked for Wilson's.

DEAR DR. DONOHUE: I hear you when you tout the use of petrolatum for dry skin itch. But I find it much too messy to use. What's next best? - Mrs. H.T.

ANSWER: Yours is a common knock on petrolatum, admittedly a good skin moisturizer. If you find it unpleasant, try any of the non-scented creams for the purpose. Individuals who find petrolatum uncomfortable when used in hot summer months often can use it in winter. It is in the winter when dry indoor heating can make dry skin worse. Humidifiers help also.

Again, let me stress the idea of water as the great moisturizer it is. The best approach is to pat dry after a shower or bath, then apply the moisturizing agent on the still damp skin. This helps retain the moisture.

DEAR DR. DONOHUE: Does breastfeeding really help prevent respiratory problems in babies? - M.I.

ANSWER: Studies done in underdeveloped countries indicate this. The protection seems to be more evident during the first four months or so. The mechanism of protection is unclear. It is probably due to antibodies in the mother's milk.

- Dr. Donohue welcomes reader mail but regrets that, due to the tremendous volume received daily, he is unable to answer individual letters. Readers' questions are incorporated in his column whenever possible.