Sexually transmitted disease (STD) rates in the U.S. are at a record high for the fourth year in a row, according to new data from the Centers for Disease Control and Prevention (CDC).

All told, nearly 2.3 million cases of chlamydia, gonorrhea and syphilis were diagnosed in the U.S. in 2017. That’s 200,000 more cases than were diagnosed in 2016, a year that also had a record-high number of cases, according to the CDC.

“We are now very concerned about this steep and sustained increase that we’re seeing,” says Dr. Gail Bolan, the director of the CDC’s Division of STD Prevention. “We’ve seen an ebb and flow of STDs in decades past, but now we’re at the highest level of our reportable conditions that we’ve seen in two decades.”

Chlamydia remains by far the most common STD in the U.S., with more than 1.7 million cases diagnosed last year. But increases in syphilis diagnoses have been particularly dramatic. Rates have risen by 76% (from around 17,400 cases to more than 30,600 cases) since 2013, according to CDC data. Bolan says that’s in part because transmission patterns are changing.

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“In past years, syphilis was predominantly among men who have sex with men, and in more recent years, we’re now seeing increases in women and heterosexuals,” she says. (Nonetheless, 70% of cases in 2017 were still diagnosed among men who have sex with men, the report says.) “A lot of communities are not aware that they’re a community with high prevalence, so we’ve got to expand our outreach and education to ensure that people are getting the appropriate screening, treatment and prevention messages.”

Gonorrhea diagnoses are also increasing and have grown by 67% (from around 333,000 cases to roughly 555,600 cases) since 2013, the report shows. Even more concerning, Bolan says, is the mounting threat of drug-resistant strains of the disease.

Gonorrhea is a “wily organism,” Bolan says. “As soon as you use one drug, it figures out ways to bypass that antibiotic. We’re now down to the last known effective antibiotic in our pipeline. It’s only a matter of time before the organism’s going to outsmart us.”

That prediction underscores the importance of developing new antibiotics and novel treatment options, such as vaccines, Bolan says. While the CDC has not seen any signs of syphilis or chlamydia developing drug resistance, she says it’s something the agency is actively monitoring. “Any bacteria, and even some viruses, can become resistant, so we’re always concerned,” Bolan says.

Bolan says the new findings emphasize the importance of any sexually active person getting tested for STDs — and practicing safe sex by using condoms — but that young women should be particularly vigilant, as 45% of chlamydia cases were diagnosed among women ages 15 to 24 in 2017. That’s in part because young women are biologically susceptible to contracting the infection, since chlamydia gravitates toward a type of cell that sits outside young women’s cervixes. Those cells migrate inside the cervix with age, and women may develop some immunity over time as well, Bolan explains.

“They have the most to lose,” she says, because these infections can lead to infertility, pregnancy complications and chronic pelvic pain.

More comprehensive community education and outreach, coupled with better screening and treatment practices from doctors — like taking detailed sexual histories from patients, and testing for STDs using more rigorous methods than simple urine tests — may help get STD rates under control, Bolan says. But the problem is a complex one, since certain communities and socioeconomic groups are more susceptible to infection than others, and other public health issues, such as substance use and the opioid epidemic, may compound the problem.

“We know that we still have a long way to go,” Bolan says.

Write to Jamie Ducharme at jamie.ducharme@time.com.