Women should carry condoms in their wallets, said Dr. Mary E. Guinan, so they might avoid pelvic inflammatory disease, a leading cause of infertility. Each year, more than 1 million American women contract the disease as a result of sexually transmitted infections.

Pelvic inflammatory disease (PID) can scar and block the fallopian tubes. Some 200,000 of those who have it will become infertile. PID also can cause ectopic, or tubal, pregnancies.

Twenty-five percent of the women with PID suffer one or more serious long-term consequences, said Guinan, associate director of the division of sexually transmitted diseases at the Centers for Disease Control in Atlanta.

Though difficult to diagnose and cure, PID is preventable. According to Guinan and other physicians, using condoms is one of the best ways to avoid this increasingly widespread disorder.

"PID has become a public health problem of alarming proportions and significant economic cost," said Dr. A. Eugene Washington, assistant director of the clinical epidemiology program at the University of California at San Francisco. "Prevention remains the most cost-effective approach to reducing the incidence of PID."

In 1984, the nation spent more than $2.6 billion on health care and lost productivity due to PID, said Washington. The costs could reach $3.5 billion by 1990, Washington estimated in a recent issue of the Journal of the American Medical Association.

PID is an infection that ascends from the lower genital tract, the vagina and cervix, to the upper genital tract, the uterus, fallopian tubes and ovaries. Normal vaginal bacteria, the use of an intrauterine device, or sexually transmitted diseases like gonorrhea and chlamydia can cause the initial infection. Scientists, however, now recognize chlamydia as the most frequent cause of PID.

Chlamydial infections, currently the most common sexually transmitted disease, afflict at least 3 million people each year. Chlamydia is five times more widespread than herpes and twice as common as gonorrhea.

Easily transmitted, the chlamydia bacteria can invade almost any part of the male or female genital tract and anus. The infection can easily be identified and treated with antibiotics, but when chlamydia causes PID, it often is not easy to eliminate.

Chlamydia can cause abdominal pain, vaginal discharge and fever. But in most cases it causes no symptons and can remain undetected and untreated. When symptoms are ignored or undiagnosed, the infection can spread, leading to PID (also known as salpingitis).

"Unfortunately, since a woman usually has no symptoms, chlamydia is a silent disease and, paradoxically, it is much more devastating than gonorrhea," said Dr. Richard Sweet, chief of obstetrics and gynecology at San Francisco General Hospital.

Chlamydial infections are most likely to occur during or just after a woman's menstrual period, Sweet and researchers at San Francisco General Hospital announced recently.

Experts are uncertain why women are more susceptible to infections during menstruation. "We don't have the answers, but those answers hold the key to how we can prevent PID," said Sweet, who added that researchers are exploring several possiblities.

According to Sweet, a mucus plug covers and protects the cervix against bacteria. This plug is gone during menstruation, which might allow organisms to migrate.

Another explanation is that the high levels of hormones during menstruation effect either the plug or the acid balance in the vagina, facilitating the spread of organisms.

Once the infection spreads, the result is a frequently painful pelvic condition. Symptoms include fever, cramps, vaginal discharge, discomfort during intercourse and lower abdominal pain which worsens over days.

Sweet advises women with these symptoms to see their physician. "All too often this gets pooh-poohed over the phone," he said.

PID is more readily treated if it's diagnosed early. Studies in Sweden show that when PID was diagnosed within two days from onset of symptoms, women did not suffer damage to their fallopian tubes. When women were diagnosed from six to seven days from onset of symptoms, 30 percent developed blocked fallopian tubes, a condition that leads to infertility.

However, PID can be difficult to diagnose, even when caused by chlamydia. Undetectable by laboratory tests, PID is diagnosed on the basis of a physician's examination.

"Even the best clinicians are only right 60 percent of the time -- it's very subjective," said Guinan of the CDC. "Women often go undiagnosed because they don't have classic symptoms. Considering that there are 1 million cases of PID each year, it's pretty frightening that we don't have a method of detection."

The only reliable way of diagnosing PID, according to Sweet and Guinan, is using laparoscopy. Usually performed under general anesthesia in hospitals, this procedure allows physicians to see inflammation of the pelvic organs by inserting an instrument through the skin of the belly.

Laparoscopy is used to detect PID in Sweden. In the United States, because PID is so widespread, such examinations would be "a logistical and economical impossibility," Sweet said.

Sweet believes physicians should be more attuned to the prevalence and hazards of PID. "Physicians need a higher index of suspicion about PID," said Sweet. "If I suspect a woman has PID but she doesn't fit all the characteristics, I often . . . assume it is -- because the option of not treating is just not worth the risk."

While PID is usually treated with antibiotics, it can be difficult to cure. According to the CDC, 15 percent of women with acute PID fail to respond to treatment and 20 percent have recurrent PID.

"When you treat PID, you don't really know when it's cured," said Guinan, who added that although 280,000 U.S. women are hospitalized for PID each year, most women don't require hospitalization. There are several ways to avoid PID. "The first thing is monogamy," Guinan said. By decreasing the number of their sexual partners, women reduce their risk of contracting diseases like chlamydia and gonorrhea. "Monogamy is the name of the game. It's a health, not a moral, issue."

For women who are sexually active with more than one partner, Guinan strongly suggests using barrier methods of contraception, like the condom and diaphragm.

"When you consider contraception, you have to consider risk of pregnancy and risk of sexually transmitted diseases," said Guinan. "If you avoid sperm deposition, you can avoid many sexually transmitted diseases. So I advise women to insist their partner uses condoms."

Guinan also recommends that all sexually active women see their physicians regularly and request screening for chlamydia and gonorrhea. "The more partners you have, the more frequently you should be checked," she said.

Other preventative measures, according to Dr. Katherine M. Stone of the CDC, include avoiding anal intercourse, and oral-anal and digital-anal activity.

Many people feel immune to sexually transmitted diseases, say CDC officials, who add that this mentality contributes to some women taking inadequate precautions and others allowing infections to escalate and cause PID.

"Middle class women feel somehow exempt from sexually transmitted disease," said Guinan. "People think somebody with a certain status doesn't have a sexually transmittable disease, but that's not true. Any man can have it, even if he's a king or priest."

Guinan and some physicians believe women must take more responsiblity in balancing their sexual life styles and their desires to have children.

"Women need a reproductive life plan -- you have to ask what you want in life and whether you want to protect your fertility," said Guinan. "Ironically, as a society, we have advocated jogging, eating well and giving up smoking, but we haven't brought that healthy outlook to our sex lives."