Writing about sex and sexually transmitted diseases (STDs) is fascinating, frustrating and fun. It's fascinating because I get to read the results of research and studies about the sex lives of ordinary people. It's frustrating because year after year I am reminded that ordinary Americans know almost nothing about STDs until they are diagnosed with one; and it's fun because when my roommate comes home and announces that he has crabs and gonorrhea, I get to decide whether or not I want to write about his little visitors and how he acquired them. I can sum up that story for you by quoting him directly: "I'm a slut."

Now let's pretend for a moment that my roommate really is a slut . . . um, I mean, well, let's just call him "sexually generous" for the sake of this article. Like a lot of people, he may not even know about the variety of sexually transmitted diseases lurking out there. So this month, let's review some of the most common sexually transmitted diseases.

Gonorrhea

A bacterial infection that can affect men or women (roughly 800,000 times a year in the U.S.). It's passed from person to person during vaginal or anal intercourse or oral sex -- so it can be found in the vagina, anus, urethra (that hole in the head of the penis) or throat. Pregnant mothers can pass it to their baby during birth, causing eye infections.

• Symptoms in men: Discharge from the head of the penis or anus; pain or itching at the head of the penis; swelling of the penis (occasionally); painful urination. Discharge from penis may happen and then stop without treatment, but you are still infected. If gonorrhea infects the throat, there are usually no symptoms other than a sore throat feeling. Gonorrhea is more likely to be spread to the throat by penis/mouth sex than by mouth/vagina sex. In the anus (butt), there may be a white discharge on stool, pain, spasm or itching.

• Symptoms in women: Discharge from the vagina; lower belly pain, especially during sex; pain during urination; unusual vaginal bleeding with cramping.

How serious is gonorrhea? Men can sometimes develop epididymitis, an infection of the testicles (balls), a painful condition that can also cause sterility. Women can also become sterile, develop pelvic inflammatory disease (PID) and manifest damage that can lead to a tubal pregnancy.

Gonorrhea should be treated with antibiotics by a doctor or clinician. Avoid having sex during treatment and for one full week after treatment. Encourage your sex partner(s) to be examined. Gonorrhea can be avoided by using a condom during anal, vaginal or oral sex. Though typically curable, some drug-resistant strains have been discovered.

Syphilis

Like gonorrhea, it's caused by a type of bacteria that can infect men or women. It's passed from one person to another during anal, oral or vaginal sex, and sometimes kissing.

Syphilis goes through four stages of infection: Incubation -- when you are first infected; no symptoms and it may take up to 90 days for a blood test to show the disease. Primary stage -- painless sores or ulcers show up on the penis, vagina, mouth or anus . . . it may go away on it's own, BUT you are still infected and can spread the disease. Secondary stage -- sores or skin rashes, especially on the hands and feet (they will go away without treatment, but you are still infected and can spread the disease). Latent stage -- no symptoms, but the syphilis bacteria remains in the body and, left untreated, can lead to heart disease, blindness, brain damage or death.

Treatment for syphilis involves antibiotic injections (typically penicillin) to kill the bacteria. You will have to repeat blood tests until cured. Avoid having sex until you are cured. According to the Centers for Disease Control, syphilis is 30 times more likely to affect black Americans than whites due to poverty, lack of access to healthcare and drug use.

Want to avoid syphilis? Use condoms and avoid having sex with partners who have sores or ulcers on the penis, vagina, mouth or butt. Less common than gonorrhea, syphilis affects over 100,000 Americans a year.

Chlamydia

With four million cases diagnosed annually, chlamydia is the number one sexually transmitted disease in America. Affecting men and women, chlamydia is a bacterial infection passed person to person during vaginal, anal or oral sex. It can also spread to the eyes by touching fluids from the vagina or penis and then rubbing or touching your eyes. Pregnant women can pass it to babies during delivery (usually showing up as eye or lung infection). Chlamydia is known as the "silent epidemic" because three quarters of the women and half of the men with the disease have no symptoms. When there are symptoms, they look like this:

• Symptoms in women: Pain and itching of the vulva (external female genitalia) or vagina; discharge or unusual bleeding from vagina or anus; pain during urination or sex.

• Symptoms in men: Discharge from the head of the penis or anus; pain or itching of the head of the penis; pain during urination.

The U.S. Preventive Services Task Force recommends that all sexually active women 25 and younger be routinely screened for chlamydia. Current CDC guidelines suggest screening women ages 20 to 24, especially if they have multiple sexual partners or do not use condoms. The American Academy of Family Physicians and the American College of Obstetricians and Gynecologists advise screening only high-risk women ("high-risk" means sexually active in this case). None of these organizations seems particularly concerned about men getting screened, something I find inexcusably backward and sexist. And remember, whether symptoms are present or not, infected individuals remain infected and can pass chlamydia to others. Left untreated, it can cause pelvic inflammatory disease (PID) or sterility in women and infertility in men.

So what's the test and treatment for chlamydia? Diagnosis (I hate this part!) usually involves a cotton swab test taken from the urethra of the penis in men (imagine a Q-Tip in the head of the penis), or from the cervix in women. Fortunately, a urine sample test is being developed. Chlamydia is curable with certain types of antibiotics. Condoms can help prevent transmission.

Genital Warts

Also known as "condyloma," genital warts are caused by the human papilloma virus (HPV). In men, warts may appear on the penis and around or inside the anus. In women, the warts may appear around the vagina or on the cervix and around or inside the rectum. So as you might have already guessed, they can be passed though either vaginal or anal intercourse. But since this virus can live on skin around the penis, vagina or anus, it's far easier to transmit than a virus such as HIV because it can be passed skin to skin during sex. Some experts claim that if you've had unprotected (no condom) sex with more than two people in your life, then you have probably already been exposed to this wart virus. And it's possible to be exposed to the virus for months or even years before warts appear.

How serious are genital warts? For most people, they're just ugly and the virus is not dangerous. However, during vaginal intercourse or butt sex they may bleed due to the friction involved. More importantly, the wart virus can cause changes in the anus or in the cells of a woman's cervix. Women should have a yearly Pap smear test even if no warts are visible and men should check regularly for warts.

Lots of treatment options exist for genital warts: topical creams, freezing, burning, laser treatment or surgery. Warts may (but not always) return because the virus remains on the skin. You can remove the warts, but you will always have the virus; there is no cure, but vaccine research is underway.

One last important note about HPV: condoms will only reduce, not eliminate, the risk of getting warts because the virus can be on the skin near the vagina, penis or rectum and not just on the sex organs. Obviously, if warts are visible you should avoid sexual contact until they're treated.

Genital Herpes

Caused by the herpes simplex virus (HSV) -- and there are two types of HSV. HSV-I usually causes fever blisters or cold sores on the mouth. HSV-II usually causes sores on the genitals (vagina, penis, anus and the skin around those areas). HSV-1 is common and most people are exposed in childhood. HSV-II infection typically happens in adolescence or adulthood. Both can cause genital herpes. Both can produce sores in and around the vaginal area, on the penis, around the anal opening, and on the buttocks or thighs. HSV-I (the mouth one) can be passed to the genitals of a sexual partner by oral sex occasionally. HSV-II (the genital one) is usually passed to the genitals of a sex partner by the infected person's genitals. It's very rare for the HSV-II to be passed to a sex partner's mouth. Unfortunately, HSV-I and HSV-II can be passed even when no actual sores are present. This happens because of something called "asymptomatic viral shedding." In other words, the virus can be present on the skin without symptoms.

Like other viruses, HSV remains in your body for life. It hibernates in nerve roots when it's not causing symptoms. Those symptoms can include swollen glands, fever and achiness in addition to blisters or open sores. The sores can last 2-3 weeks, weeping a clear secretion, then scabbing over and healing. Herpes outbreaks can happen over and over again. Research indicates that genital herpes outbreaks may be linked to stress, fatigue, lack of sleep, menstruation and genital friction. Tingling and itching of the genital area or pain in the buttocks or legs may signal an outbreak of blisters or sores. Some people will experience yearly outbreaks and others may experience them only from time to time.

Clinicians can swab an active lesion or draw blood to confirm infection with HSV. There is no cure at this point, but several drugs are available to treat genital herpes and the results have been good. Vaccines are also in development. If there are signs of a herpes outbreak or visible sores, you should avoid having sex. Condoms offer some protection, but it's best to avoid the sores until they have completely healed, not just scabbed over.

All of these sexually transmitted diseases are considerably more common than HIV. However, whether bacterial or viral, these are genital diseases with one thing in common: they increase the risk of HIV transmission.