The U.S. is in the middle of a steep and sustained increase in sexually transmitted diseases.

So how are public health officials responding?

In northwest Oregon’s Clackamas County, health officials have decided to ask anyone who comes in with an STD who their sexual partners are — and then track those partners down.

That job falls to two women: registered nurse Mary Horman and disease intervention specialist Liz Baca. They do most of the work over the phone, telling people they’ve had a partner (no name is revealed) who has tested positive for gonorrhea, HIV, chlamydia or syphilis.

It’s a difficult conversation. And many people can’t be reached via phone. So about once a week, Horman and Baca jump into a car and start knocking on doors.

Mary Horman, right, a registered nurse for Clackamas County, and Liz Baca, a disease intervention specialist for the county, search for the right address in an Oregon neighborhood. Part of their job is to get information to people who may have a serious, treatable infection, yet do not realize it. Kristian Foden-Vencil / Oregon Public Broadcasting

“It can definitely be scary at times,” Baca said, “especially those rural areas where you’re really relying on the GPS to get you there, and sometimes there are roads that lead you to nowhere.” So far, they haven’t gotten lost.

Plenty of residents in the county’s outskirts own firearms, Baca said, and are comfortable displaying them if they believe that they need to protect their property.

“I always try to make myself visible and not be fidgety,” she said. Her goal is to approach with as much warmth as possible, saying, “ ‘I have a nurse with me.’ Or, ‘My name is Liz, and I work for Clackamas County.' ”

On the afternoon I caught up with them, Horman and Baca had already been challenged by a dog and had to leave a trailer park empty-handed. They had the right location, but didn’t know which door to knock on. And they couldn’t just ask around, because neighbors might guess why the health department is tracking someone down, and that would be a breach of medical privacy.

They next called on a 64-year-old laborer, whom we’re identifying only as Larry for those same privacy reasons. He agreed to talk with them, in hopes of warning other people about the substantial risks of syphilis.

Baca and Horman explained how some diseases, like syphilis, are “reportable” — meaning that as soon as Larry was diagnosed, the doctor had to inform the county.

Larry told them that he’s already had penicillin delivered intravenously and is healing fine.

The conversation got tougher as Baca tried to understand the extent of Larry’s infection by showing him graphic pictures of sores and rashes.

“So, another symptom of secondary syphilis is the loss of hair,” she explained. “So, you suddenly lose some hair. You’re thinking, ‘What’s going on?’ In a couple of weeks, it resolves. And you’re still infected. So do you recall any of these symptoms?”

“I do,” Larry said, smiling. “And I would probably say that the lesion, I think you called it, I think occurred — it was probably about 10 years ago.”

That means Larry was living with syphilis for at least a decade — without knowing it.

Disease intervention specialist Liz Baca talks to a man who recently found out that he'd been suffering from syphilis for at least a decade. Kristian Foden-Vencil / Oregon Public Broadcasting

Many people who have syphilis are asymptomatic — meaning they are carriers of the bacteria but show no obvious symptoms. And that makes the infection tougher to fight.

Also, the fact that the disease was, for a time, nearly eradicated in much of the United States means that some younger doctors don’t readily recognize the symptoms and signs.

Larry said that he used to have sex with multiple partners he found on Craigslist, but that he can’t remember any names.

After such a long time, it’s too late to track down those people, Baca recalled, crestfallen. Still, she insisted, her time wasn’t wasted.

“Being out in the field, you find individuals and you get that public health intervention that is crucial,” she said.

David Harvey, the executive director of the National Coalition of STD Directors, said the efforts of fieldworkers like Baca and Horman are vital.

“Disease intervention specialists are doing heroic work,” he said. “They’re helping to navigate and get people into care.”

Twenty years ago, Harvey said, there were as many as 4,000 disease intervention specialists like Baca in the U.S. Now, because of public health costs, the number is down to about 1,400.

Baca said she’ll continue the work, prioritizing high-risk populations — like men who have sex with men and pregnant women.

The public health officer for Clackamas County, Dr. Sarah Present, said syphilis in newborns can cause serious neurological complications and even death.

“We have now multiple cases of congenital syphilis in our county — just in this year,” Present noted, “whereas that had been fairly unheard of for at least the last decade, if not more.”

Partly because of that surge in syphilis among babies, Clackamas now dedicates more resources to aggressively tracking down partners and encouraging testing, Present said — even if those notifications might lead to family strife.

A study released by Clackamas County earlier this summer shows that rates of syphilis have increased 1,300 percent over eight years — an increase that is so large, in part, because the number of cases involved used to be so small.

And Clackamas is no outlier. A new report from the Centers for Disease Control and Prevention found that over the past several years, the number of gonorrhea cases nationwide has increased 67 percent, and syphilis cases are up 76 percent.

The director of the CDC’s national center for STDs, Dr. Jonathan Mermin, said the nation is sliding backward.

“It is evident the systems that identify, treat and ultimately prevent STDs are strained to the near-breaking point,” he said.

Scientists say there are many reasons for the national increase — including the rise in antibiotic-resistant bacteria and the ease of finding anonymous sex in an era of cellphone hook-up apps.

As Baca and Horman visited with Larry on his front porch, the man thought back to when he first realized that he was very sick. Once he found out why his health was failing, he said, he started to take the infection seriously. “Because, really truly, for me,” he said, “it was going to be a matter of life and death.”

Getting the chance to save lives is why Baca and Horman do this work, they said, even if it’s uncomfortable sometimes.

A few minutes later, the women climbed back into the car they’d left parked in a spot where getting away would be easy — just in case Larry wasn’t so accommodating. They checked their map and the next name on their list.

“There are those areas where you’re entering a property and there’s no one in sight,” Baca said, “and dogs can be scary. You have to run very fast if you see them.”

This story is part of a partnership that includes Oregon Public Broadcasting, NPR and Kaiser Health News.

Kaiser Health News is a national health policy news service. It is an editorially independent program of the Henry J. Kaiser Family Foundation, which is not affiliated with Kaiser Permanente.