Although it is the most commonly reported sexually transmitted disease and carries serious health implications, chlamydia is still a bit of a mystery. If you've never heard of it, you're not alone.

Until a few years ago laboratories were unable to prove the existence of the chlamydia germ, and only recently have states been required to report the disease. Approximately 75 percent of women and 50 percent of men never have symptoms, so the majority of infections go unrecognized. Nevertheless, experts estimate that there are three million cases of chlamydia in the U.S. each year. Rates are currently declining due to better screening and treatment.

• Risk factors
• Signs and symptoms
• Cause
• Incubation period
• Possible health effects
• Diagnosis
• Treatments
• Follow-up
• Prevention
• Pregnancy

• Risk factors: You are more likely to get chlamydia if you are under the age of 20, if you or your partner have had many other partners, if you are using oral contraceptives, and if you have an inflamed cervix (cervicitis). Statistics show that people under age 20 more often have multiple sex partners, and that teenagers are more likely to experience a form of cervicitis called cervical ectopy. Having cervical ectopy means that thin and vulnerable layers of your cervical cells are exposed, increasing your chance of infection there. Taking oral contraceptives also causes some increase in your chances of having cervical ectopy. As with other STDs, chlamydia strikes women harder than men. From one act of unprotected intercourse with an infected partner, 40 percent of women will contract the disease while only 20 percent of men will do so.

• Signs and symptoms: The symptoms of chlamydia are similar to those of gonorrhea. Women with symptoms usually experience a yellowish vaginal discharge called mucopurulent cervicitis. Other symptoms include pain when urinating, lower abdominal or rectal pain, mucus-covered stools, intermittent vaginal bleeding, and pain or bleeding during intercourse. The lack of symptoms in many people is what makes chlamydia such a dangerous and mysterious disease. Up to 25 percent of infected men and 75 percent of infected women are usually asymptomatic, that is, they have no signs or symptoms of the disease, and can therefore spread it unknowingly.

• Cause: Although chlamydia is caused by a bacteria, the germ shares properties with viruses, including dependence on the host cell for energy and division. This combination of characteristics has made the Chlamydia trachomatis bacterium elusive to researchers, but progress has been made in developing more accurate and inexpensive tests.

• Incubation period: Signs of the disease, if any, usually show up between one to two weeks after infection.

• Possible health effects: Public health experts believe that one-quarter to one-half of the one million yearly cases of Pelvic Inflammatory Disease (PID) are due to chlamydia. PID can lead to tubal infertility and ectopic pregnancy, a potentially dangerous situation in which the fertilized egg implants in the fallopian tube instead of in the uterus. Reports from the Centers for Disease Control and Prevention show that 30 percent of women with untreated chlamydia infections will become sterile. Recent research on chlamydia-caused PID points to either an allergic response to the proteins of the bacteria or an immune response. Since chlamydia proteins and human proteins look alike, the body could mistake its own normal cells as foreign and attack them. Either reaction would cause the scarring and tubal obstruction that are the hallmarks of PID.

• Diagnosis: Because chlamydia often has no symptoms, public health officials say that all sexually active women between the ages of 16 and 25 should be tested for the infection at least once a year. Starting in 2001, managed care organizations are being graded on how well they screen for chlamydia in this group. Your doctor will suspect chlamydia if your cervix is red and swollen and bleeds easily. He or she will confirm the diagnosis with either a tissue culture or new tests that can detect chlamydia from cervical swab samples or urine. Several of these tests are so accurate that they are more reliable than a culture.

• Treatments: The recommended treatment is either doxycycline (Doryx, Vibramycin, others), 100 milligrams orally twice a day for seven days, or azithromycin (Zithromax), in a new single-dose treatment of 1 gram orally. The effectiveness of the two drugs is the same. You should decide what's best for you: Doxycycline costs only one-fourth as much, but azithromycin is much more convenient. Other treatments include ofloxacin (Floxin), 300 milligrams orally twice a day for seven days, erythromycin (PCE, ERYC, others), 500 milligrams orally four times a day for seven days, erythromycin ethylsuccinate (E.E.S., others), 800 milligrams orally four times a day for seven days.

• Follow-up: You won't need to be retested for chlamydia after you have finished your medication unless your symptoms continue or you have been re-exposed to the disease. If you do need retesting, it should be at least three weeks since you have finished the medication. Since chlamydia can also cause infertility in men, it is important that all your recent sexual partners receive treatment as well. If you are symptomatic, all partners from the last 30 days should be notified. If you are asymptomatic, it's harder to be sure who might have been infected, but you should at least notify all partners from the last 60 days.

• Prevention: Condoms can protect you from the chlamydia bacteria. Other methods, such as the diaphragm, cervical cap, and spermicides may also help protect you. It is advisable not to have sex during your treatment.

• Pregnancy: Pregnant women who also have chlamydia have been known to get postpartum endometriosis. Endometriosis is a condition in which pieces of the lining of the uterus are found in abnormal locations outside the uterus, such as the lining of the pelvis or the ovaries, bowel, fallopian tubes, and even the lungs. Pregnant women with chlamydia are also at increased risk for spontaneous abortions and stillbirths. Infants born to chlamydia-infected mothers are at greater risk for conditions such as eye infections, pneumonia, and bronchitis. Treatment for pregnant women is erythromycin, 500 milligrams orally four times a day for seven days or amoxicillin (Amoxil, Trimox, Wymox), 500 milligrams orally three times a day for seven days. Doxycycline and ofloxacin should never be taken during pregnancy, and there is still not enough research to guarantee the safety of azithromycin either.