I know that it is recommended to have an HIV/AIDS test every six months. If it is negative, does that mean you do not have it, or does it mean it just isn't showing up in your body yet? I heard that the disease can still show up later in life, as far as 10 years later?

P.R.

Someone can be infected with HIV (the virus that causes AIDS) for seven to 10 years before developing the various infections or tumors that define the advanced stage of the infection we call AIDS. But it is not true that it can take that long for the virus to become detectable. Doctors can usually diagnose HIV infection quite easily with standard blood tests within three months after a person becomes infected, and almost always within six months of infection. Newer laboratory techniques, including the polymerase chain reaction (PCR), are capable of detecting HIV in the blood only a few days after infection.

There is no recommendation that people should be tested every six months for HIV infection. In fact, people without risk factors need not ever get tested. If you have a standard HIV test (which measures antibodies the body makes in response to the virus) today, and that test is negative, then in all likelihood, you are not infected. There is a small chance that you were tested during the "window period," meaning that you were infected within the past six months but your body hasn't produced detectable levels of antibodies. But if you have not had a possible exposure to HIV in the past six months, and an HIV test is negative, then you have nothing to worry about. If you did have an exposure, such as an unprotected sexual encounter with someone you suspect may have HIV, then you should be retested in six months. If that test is also negative, then you are essentially in the clear.

I have heard many people say that we should all be tested every six months. This just does not make sense, because most people have little risk of HIV infection. People who never inject drugs and never have unprotected sex have very low risk of contracting HIV and need not get an HIV test. People who inject drugs and share needles (which they should never do) have a high risk of contracting HIV. Similarly, individuals who have unprotected sex with multiple partners are taking a big risk of contracting HIV or another sexually transmitted disease. Someone who has unprotected sex with a single partner should consider asking his or her partner to get tested.