Q. I'm 72 years old and was shocked recently when my doctor told me I had latent syphilis. I've led a good life and never suspected that something like this could happen to me.

What is latent syphilis? How does it differ from ordinary syphilis? Can you get it without sexual contact? My doctor said I would need to get three shots of penicillin. Does this sound right, even though I'm not having any symptoms?

A. By and large, syphilis is an infection you get through sexual contact with an infected person. What's unusual about this disease is that if it's not treated, it can last in the body for decades. Most people, in fact, aren't aware they have it. Yet years after first getting infected, they can develop one or more serious complications.

Syphilis is divided into four stages: primary, secondary, latent and tertiary. Primary syphilis refers to the stage when someone first becomes infected. This stage is known for causing a painless skin sore, called a chancre, at the site of contact. For men, this sore usually appears on the penis. Women may have a sore on the outside of the vagina where it's visible, or inside where they aren't aware of its presence.

Even without treatment, this sore will clear within two to six weeks. If the infection isn't treated, many people will later develop secondary syphilis. In this stage, symptoms include a skin rash, particularly on the palms and soles, and swollen lymph glands.

The germ that causes syphilis spreads throughout the body. Because of this, secondary syphilis can result in symptoms from almost any organ. The brain, liver, kidneys, eyes, intestines and joints may shows signs of infection. Earlier in this century, syphilis was known as the great masquerader, because it could mimic so many other diseases.

If secondary syphilis isn't treated, the infection enters what's known as the latent stage. This term refers to the fact that the germ lies dormant, causing no symptoms. But eventually, up to one in three people who harbor the infection will develop tertiary syphilis.

Tertiary syphilis may take decades to strike, but when it does, the results can be deadly. The brain and the heart are the two main targets of tertiary syphilis. Brain infection can lead to meningitis (a serious inflammation of the brain), dementia (which can be mistaken for early senility or Alzheimer's disease) or problems with balance, walking and bladder control.

Heart infection can damage the aorta, the main artery carrying blood from the heart. This, in turn, can lead to heart failure. Treating syphilis properly in its early stages will prevent later complications from developing. Once they develop, treatment may not fully reverse the damage done.

There are millions of people in your situation. You probably got infected decades ago and never knew it. Your doctor just happened to discover this infection on a simple blood test. In that sense, you're lucky. Treatment may prevent future problems from striking.

The standard treatment for the latent stage of syphilis is three weekly shots of penicillin. The only tricky part to this is making sure you have no signs of tertiary syphilis. To tell for sure, some doctors do a spinal tap (lumbar puncture).

In this procedure, a doctor draws off some fluid from the base of the spine to check for infection. This fluid is the same as the fluid that bathes the brain. If spinal fluid shows any signs of syphilis, the treatment is different, requiring at least 10 days of injections.

As with all sexually transmitted diseases, it's important that any sexual contacts be checked to make sure they're not infected and in need of treatment.

If not treated properly, syphilis can lie dormant in the body for decades, only to spark serious problems later in life.

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.