Sheila Loanzon was 20 years old and a pre-med student at Vassar when she learned she had herpes.

The San Jose native and her boyfriend, both virgins when they met, thought they were being careful. But she admits they were naive when it came to sexually transmitted diseases. They certainly didn’t worry about herpes until Loanzon felt sores on her genitals a few days after a sexual encounter.

Hearing the diagnosis from a campus nurse she felt she’d been branded with a 21st-century version of the scarlet A.

“I felt embarrassed, shameful and alone,” Loanzon reveals in her new self-published book, “Yes, I Have Herpes.”

The first-generation Filipina said she could no longer view herself as her “immigrant parents’ golden child with the bright smile, sparkling personality and physician future.”

For the next 10 years, Loanzon let the diagnosis erode her sense of self. Outwardly, she was successful and happy. She finished medical school and started a gynecology practice in San Jose, where she now sees up to 25 patients a day. She also dated men, then married, but neither the marriage nor the relationships worked out. She couldn’t let go of her private shame and hid it from subsequent partners.

But that’s all in the past. Loanzon, 36, no longer believes having herpes makes her a bad person — especially given that she’s far from alone. Roughly 1 in 6 people in the United States ages 14 to 49 has genital herpes. In fact, herpes affects more people than those with a less stigmatized illness like diabetes.

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With her book, subtitled “A Gynecologist’s Perspective in and out of the Stirrups,” Loanzon joins a growing number of people who have gone public with their diagnosis, through interviews, blog posts or as advocates. Like them, Loanzon is sharing her story as a way to reduce stigma and educate the public on a surprisingly common disease that has long been shrouded by stigma and misinformation.

Eliminating the stigma is necessary to promote the open, informative discussions necessary for individual and public health, says Jenelle Marie Davis, founder of the website the STD Project and spokesperson for PositiveSingles.com. After contracting herpes at 16, Davis says she, too, felt like a modern-day Hester Prynne, with some family members shunning her and school peers spreading rumors about her.

Most people don’t stress about the virus itself, she says. Herpes may be incurable but it’s not life-threatening. Many people don’t have symptoms; at most, those are mild and can be alleviated with Acyclovir and other antiviral medications.

“It’s ultimately a skin condition,” Davis says. “The physical ramifications are minuscule compared to the social ramifications, which are directly related to it being sexually transmitted.”

The public perception of herpes, from a benign “itch down there” to the scourge of a generation, changed in the early 1980s, Loanzon writes. That’s when Time magazine ran a cover story calling herpes the new “Scarlet Letter” and predicted the virus would single-handedly end the sexual revolution.

It wasn’t long before HIV supplanted herpes as the scourge, but both generated false accusations that their targets were people who were promiscuous or immoral.

Fearing rejection from potential mates for being “dirty,” Loanzon kept her secret. She hoped she was doing enough to protect her partners by regularly using condoms and avoiding sex on the rare occasions she had an outbreak.

“None of my partners said they developed lesions,” she says, “so I considered myself lucky.”

She didn’t even open up to her husband after he tearfully admitted he had genital herpes, afraid he would judge her for not being honest. She’s not sure how much their divorce was related to her secrecy. But through work with a life coach the past two years, she has come to realize the ways she wasn’t just hurting other people but jeopardizing her own future happiness by having the disease “hang over my head.”

In its “Herpes 101” chapter, which offers basics on the virus and its transmission, Loanzon illuminates how her nondisclosure and misunderstanding of the disease left her and her partners vulnerable. For example, she was infected with herpes a second time from her ex-husband because they didn’t feel comfortable talking about their respective diagnoses.

As for her college boyfriend, he assumed he couldn’t give her a genital infection because he “never had anything down there.” However, it turned out he frequently got cold sores, a common symptom of oral herpes. But he wrongly assumed he couldn’t transmit the virus because he didn’t have an outbreak at the time.

In fact, the virus can be spread through skin-to-skin contact during oral, vaginal or anal sex, even if there is no visible sore present, Loanzon says. And, yes, oral herpes can cause genital herpes.

Traditionally, genital herpes is caused by herpes simplex virus type 2 (HSV-2), whereas oral herpes comes from herpes simplex virus type 1 (HSV-1). Because HSV-1 is most often transmitted through nonsexual contact in childhood, it has become known as the “good” herpes, Loanzon notes.

But with the prevalence of oral sex, HSV-1 is involved in a growing number of genital herpes cases, says Loanzon.

While Loanzon certainly understands peoples’ fear about disclosing a diagnosis, she’s found that coming out has been mostly positive, earning her support from friends, family, co-workers and patients.

“In the two weeks since I’ve announced the book’s release on Facebook, the response has been overwhelming,” she said.

For the most part, her former sex partners also have been supportive. While writing the book, she contacted them all to let them know she had never told them about having herpes. One, with whom she had a tumultuous relationship, was angry. He told her it was “messed up” that she didn’t tell him, especially as she was a doctor.

But others thanked her. “So far I’m good,” one said, adding that herpes “is pretty common and not a big deal as long as you take care of it.”

She views her diagnosis as a profound experience that she chose to grow from: “There is power in telling others the truth. There is power in having choices.”