DEAR DR. DONOHUE: Our daughter has come down with mono. We have other children, and we understand the mono virus can linger a long time. Can you say about how long and how we can protect the others? - Mrs. L.M.A.

ANSWER: When one family member has mono, there's a 40 percent chance that others who have not been exposed in the past will catch it. Strangely, most people don't get mono from a person with current symptoms, but rather from someone who has had it in the recent past.The reason for this late transmission is that the virus can remain in saliva and be active as long as 18 months after the person has recovered from the infection.

All you need do is keep drinking glasses and silverware separated. Even with this, you aren't going to improve greatly on the 40 percent transmission odds.

DEAR DR. DONOHUE: I am trying to visualize what a Lyme disease rash should look like. I know it consists of circles, but that's all. - Mrs. J.M.

ANSWER: The typical Lyme disease rash looks something like a shooting target - a red center and pinkish surrounding circle with a darker rim. The rash can appear within a few days or up to 30 days after exposure to the infected tick. The target-circle rash can appear in other parts of the body with time. Some people may have no rash at all, but may have other symptoms, including joint pain, fever, headache, gland swelling and fatigue.

DEAR DR. DONOHUE: Your recent article on genital herpes interested me. Though I had never been unfaithful to my wife in 20 years, I contracted genital herpes 20 years ago. I had about a year of recurrent attacks. A friend suggested I got herpes because I had not been circumcised. My question is, if I had been circumcised as a baby, as many are routinely, would I have avoided genital herpes? - N.F.

ANSWER: I have not seen any studies comparing the rate of genital herpes in circumcised and uncircumcised men. I don't think there would be much difference, if any. Recurrences occur because we have no medicine to get rid of the virus completely. Acyclovir reduces the number of attacks and does hasten healing when one occurs. The herpes report will answer your other questions. Others can order it by writing: Dr. Donohue/No.17, Box 830, Gibbstown, NJ 08027-9909, enclosing a long, stamped, self-addressed envelope and $2.

DEAR DR. DONOHUE: I have a brother who has been on prednisone for some time. He has arteritis. Can you tell me the difference between that and vasculitis? - M.S.

ANSWER: Vasculitis is inflammation of blood vessels. Arteritis is a form of vasculitis affecting the arteries.

There are many kinds of arteritis - polyarteritis nodosa and giant cell arteritis, etc. Has your brother's doctor specified the type? Cortisone (prednisone, for example) is often used to control blood vessel inflammation. It's usually quite effective.

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.