DEAR DR. DONOHUE: My 13-year-old son has mononucleosis, and we don’t know much about it. How long should we keep him out of school, and when can he play sports? He has not been near anyone who was sick for the past month, so where did he catch it? – K.V.

ANSWER: You won’t likely be able to trace your son’s source of infection. Mono is primarily spread through saliva, and that’s how it got the name of “kissing disease.” Sharing eating utensils or drinking glasses can also transmit it. Healthy people who had mono in the past can continue to spread virus for a long time, and that makes identification of the source most difficult.

Close to 90 percent of adults have evidence of having been infected with the virus. That gives you an idea of its ubiquity. It can strike at any age, but the peak years for infection are those between 14 and 24.

Prominent symptoms include sore throat, enlarged neck nodes and fever. Fatigue is so overwhelming that mono patients put up no argument about going to bed. Sometimes a rash appears on the palate. For reasons I cannot explain, symptoms in young children are less severe than those in older children and adults.

Most people can resume activities, including school, in two weeks. They should not lift heavy loads or play contact sports for four to eight weeks. In mono, the spleen enlarges and becomes mushy. It can rupture from heavy lifting or from a direct blow to the abdomen.

The virus that causes mono is the Epstein-Barr virus. As with so many viral diseases, there is no medicine that can put an end to the Epstein-Barr virus or shorten the length of infection. There are, however, medicines that make patients more comfortable.

DEAR DR. DONOHUE: I am 82 years old, and I have Kaposi’s sarcoma. Doctors tell me it is found in people who have AIDS. I have not been sexually active since my wife died more than 10 years ago. How could I get this, and what is the best treatment for it? – H.D.

ANSWER: Before the AIDS epidemic, Kaposi’s sarcoma was a seldom-seen skin cancer confined mostly to older Jewish, Italian or Greek men. Since the AIDS epidemic, it has become a fairly common skin cancer seen in people whose immune systems have been devastated by the AIDS virus.

The cancer appears as purplish skin patches. If you saw the movie “Philadelphia,” the purple blotches on the Tom Hanks character were Kaposi’s sarcoma.

The herpes-8 virus causes this cancer. That is not the herpes of cold sores or genital lesions. The way it spreads to people is somewhat of a question mark.

In people who do not have AIDS, this cancer is a very slow-growing tumor, and it also spreads very slowly. This is the kind of Kaposi’s sarcoma you have. In your case, there are many options open for treatment, including surgery, chemotherapy and radiation. There is no one best treatment. The method chosen is dictated by the circumstances that surround your particular case.

These remarks do not apply to AIDS-associated Kaposi’s.

DEAR DR. DONOHUE: Can sweating, physical exertion or taking medicine following a flu shot decrease or eliminated the effectiveness of the shot? – G.Z.

ANSWER: None of the above lessens the immunizing effectiveness of a flu shot.

DEAR DR. DONOHUE: I have bulimia and cause myself to vomit about 10 to 15 times a month. I wonder how that affects my health. – K.M.

ANSWER: Bulimia (also known as bulimarexia or bulimia nervosa) is the eating disorder where a person induces vomiting or takes large doses of laxatives to prevent weight gain. Self-induced vomiting can destroy teeth, inflame the esophagus, cause salivary glands to enlarge or cause a highly destructive pneumonia if gastric contents trickle into the lungs when vomiting.

You need help, and your family doctor can tell you where that help is available.

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.