DEAR DR. DONOHUE: I know this isn’t flu season, but I have some flu questions I’d like to get the answers to. When’s the best time to get a flu shot? Two years ago, after getting the shot, I got the flu. How do you explain that? How long is a person who has the flu capable of giving it to others? How is it spread? Why don’t you get it in spring and summer? — H.M.

ANSWER: Late fall throughout the winter and early spring is the flu season. I don’t know why spring and summer are usually free of it. In the tropics, flu is a year-round affair.

The best time to get a flu shot is from mid-September through March. It takes a couple of weeks for the body to make antibodies against the flu virus. Antibodies are ammunition against infections.

Getting the flu from a flu shot is impossible; those vaccines contain dead flu virus. How you got the flu after the vaccine might be that you had not yet developed antibodies so you were still vulnerable to live flu virus from infected people. Or you might have had one of those illnesses that people call flu but are not really influenza. “Intestinal flu,” for example, is a misnomer. Flu causes respiratory symptoms like cough along with achy muscles, a rise in temperature and a total loss of energy. It does not cause nausea, vomiting or diarrhea.

Flu is spread via droplets from the respiratory tract, through coughing, sneezing or even talking. An infected person is most likely to spread the virus from one day before flu symptoms appear to around day six of the infection. What made you pick on flu questions at this time of year?

TO READERS: Electrolytes, sodium, potassium, chloride and bicarbonate are a subject of mystery to most people. The booklet on those substances explains them and the role they play in health and illness. To order a copy, write to: Dr. Donohue — No. 202, Box 536475, Orlando, FL 32853-6475. Enclose a check or money order (no cash) for $4.75 U.S./$6 Can. with the recipient’s printed name and address. Please allow four weeks for delivery.

DEAR DR. DONOHUE: My daughter is on a diet where she eats only proteins or carbohydrates at a meal — she never eats both at the same meal. She says that eating both multiplies the calories you’re taking in, and you gain weight. Is there any basis for this? — B.W.

ANSWER: Not that I know of. That’s the weirdest diet I have ever heard of. It makes no sense. The multiplication of calories is bunk.

DEAR DR. DONOHUE: I am in my 80s and had a chest X-ray recently. The report said I have aortic calcifications. I wonder what the prognosis of that is. Do I need to worry? — M.P.

ANSWER: Practically everyone your age would see the same words on a report of their chest X-rays. Calcifications of the aorta are expected at 80.

As long as your doctor hasn’t made an issue of them, you do not need to worry.

DEAR DR. DONOHUE: My doctor tells me I have a one-degree heart block. If it turns into three-degree, I will need to have a pacemaker. Does this always happen? — H.P.

ANSWER: You have a first-degree heart block, and it is not a health threat. It produces no symptoms. It’s discovered only on an EKG. It’s a common finding for which nothing need be done.

A third-degree heart block indicates that there is no passage of the electrical signal from the upper heart chambers to the lower chambers. The electrical signal is responsible for every heartbeat. That’s why a pacemaker is needed. First-degree heart blocks rarely, if ever, become third-degree blocks.

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