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Although sexually transmitted infections are still at epidemic levels in the U.S., scientists at the U.S. Centers for Disease Control and Prevention are expressing cautious optimism about some declines emerging in the latest data.

Overall, CDC scientists say, there is evidence that the nation’s STI epidemic may be slowing, particularly when it comes to gonorrhea and syphilis, but there is still a long way to go.

“I’m encouraged and it’s been a long time since I felt this way about STIs,” said Dr. Jonathan Mermin, director of CDC’s National Center for HIV, Viral Hepatitis, STD, and TB Prevention. “However, there were still more than 2.4 million STIs reported in the US in 2023, and the impact of STIs on the health of Americans remains severe.”

There are more than 30 types of bacteria, viruses and parasites that people can pass on to their partners while having vaginal, anal or oral sex, the World Health Organization says. Some of these infections can also be passed from a pregnant person to their baby, sometimes with fatal results.

Globally, 1 million curable STIs are passed to a partner or child every day, According to WHO. STIs can also cause health problems beyond the initial symptoms; some can lead to reproductive problems and even organ damage if left untreated.

A CDC report released Tuesday tracks three nationally notifiable STIs — chlamydia, gonorrhea and syphilis — and finds that more than a million more cases were reported annually in the U.S. in 2023 than 20 years earlier.

Chlamydia, a bacterial infectionwas the most common STI in the new report.

The number of chlamydia cases reported in the US in 2023 – more than 1.64 million – was about the same as the previous year. There was a slight increase in the rate of cases among men and a slight decrease among women.

Teenagers and young adults consistently account for more than half of chlamydia cases.

One of the biggest challenges with chlamydia is that there are often no symptoms, or an infection may have minor signs that are shared with other diseases, such as a burning sensation when urinating or abnormal discharge. However, without treatment, chlamydia can cause problems that make it difficult to get pregnant later.

A person with chlamydia can be treated with antibiotics, and condoms can prevent infections.

The CDC recommends that sexually active gay and bisexual men be tested for chlamydia every year. The agency also recommends annual testing for women who are sexually active and younger than 25, or who are older than 25 and have risk factors, such as multiple sexual partners.

There were 601,319 cases of bacterial infection GONORRHEA reported in 2023, the second consecutive year of declines.

The gonorrhea case rate decreased by 7.7% from 2022 to 2023 and by 9.2% from 2021 to 2022. The decreases were seen across all demographics.

As with chlamydia, gonorrhea may not cause symptoms, but an untreated infection can lead to fertility problems. It can be prevented by using condoms and treated with antibiotics.

Doctors recommend annual testing for all sexually active women age 25 or older with multiple sexual partners, and the CDC suggests sexually active gay and bisexual men get tested every year.

poxa bacterial infection that can affect the brain, nerves, heart and eyes if left untreated, has been a growing challenge for health care providers in the United States. The country has seen double-digit increases in case rates in recent years, but in 2023, rates have risen only slightly to 1%.

The total number of syphilis cases reported in 2023 was 209,253, the highest number since 1950. The new report notes that there were significant decreases in people in the first two stages of the disease, which are more contagious than the later stages.

Syphilis is spread when a person comes into contact with a syphilis sore, and not all sores can be covered by a condom, so condoms don’t prevent all cases.

The The CDC recommends that people who are sexually active and identify as gay or bisexual men get tested regularly. The agency also suggests testing people who have HIV, people who take PrEP to prevent HIV, those who live in a community with high rates of syphilis, or those who have a partner who has tested positive for syphilis.

Pregnant people can transmit syphilis to the fetus, for which an infection can be extremely dangerous, possibly leading to stillbirth, premature birth or even death of the newborn. Also, all pregnant women should be tested at their first prenatal visit. If they have an infection, antibiotics can prevent the disease from spreading to the fetus.

The number of congenital syphilis cases for 2023 was the highest since 1992, with 3,882 cases reported, including 279 stillbirths and neonatal/infant deaths. However, the rate of congenital cases shows signs of slowing, with an increase of just 3 percent between 2022 and 2023. By comparison, previous years saw increases of up to 30 percent, the CDC said.

Researchers behind the new report say they can’t explain exactly why so many people have STIs, although the U.S. epidemic follows a global trend.

The CDC’s Mermin believes there are several factors driving STI trends, including a concerted effort among leaders in the public health community to diagnose and treat diseases early to help control the spread. He also says the growing number of people taking PrEP to prevent HIV may be driving the number down because get tested regularly for STIs.

In addition, Mermin noted an “unprecedented” temporary infusion of funding for disease intervention specialists at public health departments in the Biden administration. There is no clear indication that such funding would continue or increase under a second Trump administration, and funding varies at the local and state levels.

“It’s important to put our resources where our epidemics are, and STI prevention is a component of public health,” Mermin said.

The figures in the report reflect what Dr. Philip Chan at his own clinic, Health open doorthe only publicly funded STI clinic in Rhode Island. While the improvement in disease rates is encouraging, he said, good numbers a year from now don’t mean the epidemic is over.

Chan, associate professor of behavioral and social sciences and conf. university of medicine at Brown University, said he believes increased testing and treatment disrupts the transmission of STIs in the community. He also points to other positive trends, such as a drop in the number of high school students who have four or more sexual partners and a 10 percent drop in the number of high school students who say they have sex.

“It’s good news, and we know that regardless of your views on sex, later onset of sex is generally associated with better health outcomes,” said Chan, who was not involved in the new report.

For this positive trend among STIs to continue, he said, what would help most is something that would help the health of Americans in general.

“I think the biggest thing we could do when we talk about STIs is just improve overall access to primary care and health in general,” Chan said. “Access to care is probably the most important thing.”

CNN’s Deidre McPhillips contributed to this report.