Q. So much has been written about how to protect yourself from AIDS. My question is, how do you know when you should get a blood test for AIDS? What are its symptoms? How do you suspect you have this illness before reporting to a doctor or clinic for tests?

A. There are really two answers to your question. One involves when to consider getting a blood test to check for infection with the AIDS virus, even if you don't have any symptoms. The other involves the symptoms that might make you first suspect this serious illness and see your doctor.

The Centers for Disease Control recommends that people from groups at high risk of infection with the AIDS virus get a blood test to check for evidence of exposure. The AIDS virus -- known as human immunodeficiency virus (HIV) because it triggers illness by crippling a person's immune defense system -- is spread in three ways: by sexual contact, exposure to infected blood or blood products, and from an infected mother to her unborn child.

Because of the way this virus is transmitted, high-risk groups include homosexual or bisexual men (66 percent of U.S. AIDS cases), intravenous drug users (17 percent), homosexual or bisexual men who also are intravenous drug users (8 percent), recipients of contaminated blood transfusions (3 percent -- virtually all before 1985, when donated blood was first tested for AIDS infection), and children born to mothers infected with the AIDS virus (1 percent).

Ninety-three percent of AIDS victims in this country are men. Now, heterosexual transmission of AIDS, although still infrequent (about 4 percent), is increasing. So sexual partners of persons from any high-risk group must also consider themselves at risk of AIDS infection. For people outside these groups, the incidence of AIDS infection is extremely low, about one per 100,000.

If the test result is "positive," it means the body has formed antibodies to the AIDS virus -- a sign of infection. It is estimated that 20 to 30 percent of infected people will develop AIDS within five years, and many experts believe the percentage will increase over time.

Is it helpful to know if you are infected? That is an individual decision. I think that if people know they are infected, they can be especially careful not to transmit the infection to others. And people who know they are free of the AIDS virus can be especially careful to keep themselves protected.

The symptoms of AIDS itself are the symptoms of the diseases, primarily infections, associated with this condition. AIDS weakens the immune system and allows unusual infections and diseases to set in, which ordinarily would not develop.

Examples of these infections are those caused by parasites (pneumocystis carinii pneumonia), fungi (thrush), bacteria (tuberculosis) and viruses (cytomegalovirus infection). In addition, AIDS is also known for causing a generally rare form of cancer, Kaposi's sarcoma.

Because so many illnesses can be associated with AIDS, there isn't one group of symptoms that's distinctive for this condition. However, the following are symptoms that occur commonly in AIDS victims: unexplained fevers, night sweats, fatigue or weight loss; persistent diarrhea; chronic cough; shortness of breath; dark skin bumps usually starting on the feet and lower legs; swollen glands in the neck, groin or under the arms; and impaired mental function.

Any of these symptoms in themselves would be enough reason to see your doctor; if you suspect you might have been exposed to the AIDS virus, that's all the more reason to get an examination and blood test.

Jay Siwek, a family physician from Georgetown University, practices at the Fort Lincoln Family Medicine Center and Providence Hospital in Northeast Washington.

Consultation is a health education column and is not a substitute for medical advice from your physician. Send questions to Consultation, Health Section, The Washington Post, 1150 15th St. NW, Washington, D.C. 20071. Questions cannot be answered individually.