DEAR DR. DONOHUE: Is there an age when one is more or less likely to get flu? I used to get it in my younger years, but have been lucky since then, I guess. Last year, I had terrible intestinal sickness, but my doctor said it wasn't flu at all. My husband said it was flu, so who's right? I think everyone's confused about flu. - Mrs. I.D.H.

ANSWER: Flu is a respiratory illness, not an intestinal one. It comes on quickly. You can be feeling fine one minute and then be sick from it, with runny nose, muscle aches, headache and cough developing quickly.Your first question is an interesting one. Those who get sickest from flu are the very young and the very old. Why? We believe the young are prone because their bodies have not yet been exposed to the many flu viruses. As they get older, continuing exposure has produced various antibodies that confer a degree of immunity during flu outbreaks.

Older people in general face challenges because time has a way of weakening the natural antibody protections they enjoyed in middle years. Flu tends to be more threatening to them, much as it is for youngsters. So the short answer to your question is that the years between youth and old age are probably the best insofar as natural flu protection is concerned.

DEAR DR. DONOHUE: My granddaughter is 5. She lives in Canada, and I only get to see her once or twice a year. When she comes, I notice she is coughing. I learned that she was born with heterozygous antitrypsin deficiency. Can you please tell me what that is and what causes it? What can be done for it? - Mrs. J.T.

ANSWER: Let's start with trypsin, the key root. Trypsin is a body cleanup substance released by cells to mop up debris left behind in the fight against an invader: germ, chemical or other.

Antitrypsin is a companion substance. Its job is to turn off the cleanup operation. Without it things get out of control. In a sense, the over-cleaning damages viable lung tissue. Thus is created a somewhat rare form of emphysema.

The term "heterozygous" refers to inheritance. To reach the emphysema stage, one must have must have inherited two genes for it, one from each parent. That's the homozygous state. If only one gene is inherited (the heterozygous state) antitrypsin is still made, but in rare cases there may be a tendency toward some lung damage.

I doubt that you can lay your granddaughter's cough to her heterozygous status for antitrypsin deficiency. If there is any feeling that she does lack antitrypsin (an enzyme), that can now be supplied through injections. What does her doctor say about all this?

DEAR DR. DONOHUE: What about vitamin D supplements and osteoporosis? - Mrs. W.K.

ANSWER: No one can say for sure that D supplements will help prevent or reverse osteoporosis. About all that can be said is that the recommended daily D need is 25 micrograms. If you don't get that amount from food or sun exposure you may need supplements.

- DR. DONOHUE'S BOOKLET NO. 23, "Osteoporosis, Prevention and Treatment, " explains this debilitating disease and what you can do about it. For a copy, write Dr. Donohue/No. 23, P.O. Box 830, Gibbstown, NJ 08027-9909. Enclose a long, self-addressed, stamped envelope and $2.00.

- 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.