I have had sexual intercourse with someone who has the hepatitis B virus. We only had unprotected sex once. What are the chances of me getting hepatitis B? When should I get a blood test for it? He contracted the virus through a blood transfusion. After that, he and his ex-wife had a child, and both the ex and the child are negative. Still, I'm worried about my own health. Also, what chance do I have of exposing someone else to it?

Debbie

Hepatitis can be caused by at least five different viruses, conveniently named hepatitis A, B, C, D and E. There is also a G, but no one really knows whether it causes disease in humans. The typical ways of getting hepatitis B include sex, sharing bloody needles and blood transfusion (which is rare today because of testing of donated blood). The disease can also be passed from a pregnant woman to her baby in the womb or during childbirth.

Although hepatitis B is transmitted much the same way as HIV (the virus that causes AIDS), hepatitis B is more contagious. For instance, if a healthcare worker pricks herself with a needle contaminated with HIV, the risk of infection in the worker is about 1 in 300. If she pricks herself with a needle contaminated with hepatitis B, the risk of infection is about 1 in 3.

In an adult, hepatitis B usually causes a liver disease called acute hepatitis. The patient feels feverish and fatigued. He or she may have pain on the right side of the abdomen, where the liver is, and jaundice (yellowing of the skin and eyes). On the other hand, some people never know they have been infected.

In about 90 percent of cases in adults, the body gets rid of the infection within six months. In about 10 percent of adults -- and in many, many newborns who acquire the virus at birth -- the body never clears the virus. These patients are known as "chronic carriers." Some have ongoing liver disease, called chronic hepatitis, while others just harbor the virus without having disease. Either type of carrier is highly likely to spread the virus to others.

You need to find out if your sexual partner is a chronic hepatitis B carrier. This can be quickly and easily determined by a blood test. If so, your risk of hepatitis B from a single sexual exposure is still not very high. Even if you do get the disease, it will most likely be mild and go away by itself.

You may be able to prevent the infection if your exposure was less than a week or so ago. Two types of injections are needed to prevent hepatitis B after exposure. First, your doctor should immediately give you an injection of hepatitis B antibody. Antibodies are molecules created by the immune system to fight a specific organism. This injection, called HBIG for "hepatitis B immune globulin," is safe and effective. Your doctor should also give you the hepatitis B vaccine. The vaccine is also safe, and it is almost 100 percent effective in preventing future disease. In fact, the vaccine is now part of the standard immunization series for all children born in the United States. You should receive the vaccine in three shots given one month apart.