Herpes. It conjures up images of dirtiness and immorality. The word itself is Greek for snake.

But the herpes virus is among the most common and nonthreatening known to medicine, according to medical experts. Between 70 and 90 percent of the world's adult population has contracted herpes simplex I, which commonly produces cold sores, and they often do not know it.

Although it seems to be a new concern, herpes has been around since ancient times. In the first century, the Roman Emperor Tiberius banned kissing at public ceremonies because of an epidemic of cold sores.

Herpes is actually a family of four viruses. The most common, herpes simplex, is at the heart of the current national debate. It falls into two types: Type I, in 85 percent of the cases, occurs above the waist; Type II, or genital herpes, occurs below the waist most often, said Dr. Ward Cates of the U.S. Centers for Disease Control in Atlanta.

Either type of disease, when in a mother's birth canal, can blind and cause nerve damage to an infant during birth. Adults and children beyond the first six weeks of life who have eczema or weakened immune systems also can be harmed by the virus, but "the great majority of us, we get lesions that are annoying rather than terrible," said Dr. Mervyn Elgart, chairman of the dermatology department of George Washington University medical school.

The lesion resembles a mosquito bite, which may rupture or be absorbed back into the body. Only when lesions are present is the disease infectious, experts said. No sexual contact is necessary to transmit either type of the simplex virus, Elgart said. Some people have herpes and never break out in lesions, he added.

Herpes simplex, and a herpes virus responsible for chicken pox and shingles, distinguish themselves from other viruses by remaining in the body's nerve cells indefinitely. Lesions may recur throughout a carrier's lifetime; however, as its sufferers age, breakouts become less frequent.

Cytomegalovirus, a third herpes virus, produces mild flu-like symptoms and no lesions. It commonly goes undiagnosed and poses no health threat except to young infants and fetuses.

Epstein-Barr, the most uncommon of herpes viruses, usually produces infectious mononucleosis in this country, but it has been connected with Burkitt's lymphoma among African children. It is the least understood of the four strains, Elgart said.

The key to understanding the fear surrounding herpes, suggested Elgart, is that it is not curable. "But it's the kind of thing that at its worst can still be taken care of," he said.