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(CNN) —  

America is in the middle of an epidemic of sexually transmitted infections, and when it comes to heterosexual transmission, it’s hitting women the hardest.

Why is that?

Simply put, because “STDs are biologically and psycho-socially sexist at all levels,” said Dr. Hunter Handsfield, a professor emeritus of medicine at the University of Washington Center for AIDS and STD who has studied sexually transmitted diseases for 40 years.

“Women bear the largest burden of these diseases,” agreed Dr. Edward Hook, co-director of the Center for Social Medicine and Sexually Transmitted Diseases at the University of Alabama at Birmingham.

“Chlamydia and gonorrhea, for example, are two of the leading preventable causes of infertility and ectopic pregnancies in the United States, and on Earth.”

Easily transmitted, more dangerous

One reason STDs are sexist: In heterosexual pairings, they are more readily transmitted from the man to the woman than from the woman to the man.

“So at any one exposure, a susceptible woman is more likely to catch it than a susceptible man,” Handsfield said. That’s because the lining of the vagina is thinner and more delicate than the skin on a penis, so it’s easier for bacteria and viruses to penetrate and take hold. Once there, the moist environment of the vagina is perfect for growth.

And once the infection is acquired, it could be more severe or more damaging to a woman’s health. Take herpes, for example. “Because of the nature of the female genitalia, if you get new herpes, there’s likely multiple painful blisters,” Handsfield said. “For men, typically a few blisters.”

Another example is human papillomavirus, or HPV. It causes cervical cancer, which can be deadly unless caught early.

“Yes, men can get penile cancer from HPV,” Hansfield said, “but at about one hundredth the rate that women get cervical cancer from HPV.”

According to the World Cancer Research Fund, cervical cancer is the eighth most common cancer in the world.

Hidden in the female experience

Yet another reason STDs are sexist: Symptoms are often more nonspecific in women, and can be mistakenly written off as a typical female annoyance.

“If a woman has a little burning on urination, maybe it was too much time on the exer-cycle, maybe the underwear was too tight, maybe there were spicy foods for dinner,” said Hook.

“Maybe there’s a little discharge, a little itch,” Handsfield said. “It’s easy for a woman to think it’s a yeast infection and self-treat while the chlamydia is climbing up into her fallopian tubes.”

If there are any signs at all. Many men and women with STDs never have symptoms and have no idea they are infected.

Harder to diagnose

The sexist nature of STDs can even follow a woman into the doctor’s office.

“If a guy goes into a doctor’s office with a urethral discharge,” Handsfield said, “there are only a very small handful of things that can cause it, and most of those are STDs. Any half-trained medical school student can diagnose it and accurately get it treated.”

A woman’s discharge or uncomfortable urination, however, could be due to a number of reasons, including a urinary tract infection. Doctors often need urine samples or must swab the vagina and send samples to a lab for analysis, which might take days to come back and are sometimes inconclusive.

“Even the most sophisticated expert clinicians end up scratching their heads about the cause of 20% or 30% of women’s vaginal discharge,” Handsfield said.

And then ….

Sometimes, it’s more than the STD that’s sexist.

“My wife is a OB-Gyn and I am an infectious disease physician who specializes in STDs, so there’s been a lot of talk about this issue at my house,” Hook said.

“Once my daughter went to see her doctor and at the end of the exam the doctor said, ‘Okay, you’re good to go.’

“Now for over 10 years, it’s been recommended that every woman age 24 or less who is sexually active be tested for chlamydia once a year. But data show that occurs in less than 50% of women in our country,” he explained.

“So my daughter said, ‘Well, did you do a chlamydia test?’ And that elicited a lecture from the doctor on why she didn’t need to have sex to please boys.”

Most cases ever recorded

Cases of gonorrhea, chlamydia and syphilis have reached the highest levels ever recorded, according to a recent report from the US Centers of Disease Control and Prevention.

“Combined, they total 2.4 million infections that were diagnosed and reported just in last year alone,” CDC epidemiologist Elizabeth Torrone told CNN when the report came out.

The latest numbers show youth, especially girls, are most at risk. The CDC estimates people ages 15 to 24 acquire half of all new STD cases, while one in four sexually active adolescent girls has an STD.

“These numbers, astounding as they are, probably represent only half of all STD cases because they are not uniformly reported to the CDC,” Hook said.

Some of the increase in numbers may be due to new and improved tests that are more sensitive to spotting disease, as well as the testing of orifices doctors ignored in the past.

“With the popularity of anal and oral sex, we now look at what we call extra-genital sites of sexual exposure, both the throat and the rectum,” Hook said.

Another key reason for the skyrocketing numbers is a drop in condom use. That trend is a critical mistake, say experts, because barrier protection is one of the only ways – besides abstinence – to keep STDs at bay.

What’s not a reason for the high rates of STDs in women, experts say, is the sexist attitude that women who get these diseases must somehow be more sexually promiscuous than men.

“There are 1.7 million cases of chlamydia in the United States,” Handsfield said. “And we know that rates are higher in women than men. And we also know more than half of the women who get infected with chlamydia had only a single sexual partner in the last year.”

What women should do

Take away the guilt. “I think that most people believe that the only people who get sexually transmitted infections are people who are somehow taking risks and doing wrong things,” Hook said. “And that’s clearly not the case.”

Know your body. “Learn your genitals,” Handsfield said. “Learn what’s normal for you and be aware of any new oddities, and don’t be afraid to check them out.

“Any new or unexplained vaginal discharge or change in the character of vaginal discharge, including odor, requires professional evaluation,” Handsfield said, adding that any unexplained genital sore, lump or swollen gland in the groin, low abdominal or pelvic pain, or change in menstrual pattern are all reasons to visit a doctor.

“Spotting between periods and spotting after sex is high risk of an inflamed cervix due to STDs,” Handsfield said. “Many women have symptoms like this from time to time anyway, so it’s a bit frustrating, but you’ll know if that is abnormal for you.”

Get tested often. “Everybody owes it to themselves to make sure they’re sexually healthy and they shouldn’t hesitate to be tested for these infections,” Hook said. “Look at it this way: If your tests are positive, the good news is that you found the infection and it can be treated before it causes you problems or gets transmitted to others.”

CNN’s Jacqueline Howard contributed to this report.