''Each year, another half million cases of genital herpes are added to the 20-odd million already infected,'' notes Dr. Fred Rapp, a microbiologist at Hershey Medical School and a leading herpes investigator. ''We will soon have 30 million cases, and the chances of getting infected by an occasional sexual partner keep increasing.''

According to Dr. Rapp, other characteristics responsible for the increased notoriety of herpes viruses and the difficulties researchers have in combating them include the following:

- All the herpes viruses can hide out in the body, apparently for life. At least three - the cold sore, genital herpes and chicken pox viruses - can re-emerge to cause recurrent infections.

- Although the first effective antiviral drug was developed against a herpes virus, there is still no cure for human herpes and no treatment can eradicate hidden infections or prevent their recurrence. Treatment can ward off herpes-caused blindness and potentially deadly encephalitis, but in the latter case, the treatment itself can have devastating side effects.

- All five human herpes viruses have been shown in the laboratory to transform normal human cells into cancer cells, though proof that the viruses actually cause cancer in people is still indefinite. Two nonhuman herpes viruses are already established causes of cancer in animals: Marek's disease, a lymphoma of chickens, and kidney cancer in leopard frogs. In addition to cervical cancer, in people herpes viruses are suspected causes of Burkitt's lymphoma, nasopharyngeal carcinoma, and possibly also vulvar cancer, Kaposi's sarcoma (which occurs primarily among homosexual men) and cancer of the prostate and bladder.

- Pieces of genetic material from genital herpes virus have been found in cancer cells of 40 to 50 percent of patients with cervical cancer, Dr. Rapp said. One study showed that women who married men whose first wives had cervical cancer themselves faced a three-tofour-times increased risk of developing the disease.

- Vaccines, which stimulate the production of antibodies to fight off infectious organisms, may not block latent or recurrent herpes or prevent herpes-triggered cancers. Recurrent infection and herpesassociated cancers occur in people who are loaded with antibodies, which may keep the virus from causing a body-wide attack, but cannot prevent recurring herpes lesions. Nor would they keep the virus from transforming cells to cancer.

However, a successful vaccine was developed against Marek's disease of chickens, raising the hope that similar preventives might ward off initial infections with human herpes viruses.

When the virus invades a cell, it sheds its outer layer, allowing viral genes to take over the cell's genetic machinery and turn it into a virus factory. The herpes viruses are structurally very similar, but most differ widely in genetic makeup. Only the two herpes simplex viruses share a large percentage of genes.

''The herpes viruses are genetically the most complex human viruses we have left to deal with,'' Dr. Rapp said. Yet only a small amount of viral genetic information - probably less than 3 percent of the total in a herpes virus - is needed to transform cells and maintain their cancerous state, Dr. Rapp has reported. Researchers working on herpes vaccines must be certain that the viral material used in the vaccine cannot itself transform cells to cancer. The usual vaccines made from modified live viruses probably could not be used against potentially cancer-causing herpes viruses.