Sexually-transmitted diseases (STDs) are on the rise in the U.S. for the fourth year in a row, according to the latest data the Centers for Disease Control and Prevention (CDC) released Tuesday.

The CDC announced Tuesday that 2.29 million cases of chlamydia, syphilis, and gonorrhea had been diagnosed in the U.S. throughout 2017—an increase of more than 200,000 cases over 2016.

The rise in STD cases is part of a “steep and sustained increase” since 2013. Reported cases of three well-known STDs all increased between 2016 and 2017.

Cases of chlamydia increased by nearly seven percent, primary and secondary syphilis cases rose by more than ten percent, and gonorrhea cases soared by nearly 19 percent over the past year, according to the CDC data.

“We have seen steep and sustained increases over the last five years,” Dr. Gail Bolan, the CDC’s director of the Division of STD Prevention, told NBC News. “Usually there are ebbs and flows, but this sustained increase is very concerning. We haven’t seen anything like this for two decades.”

Experts say there are a number of factors that have caused the spike in STD cases, but many experts point to less frequent condom use as one of the main reasons.

It is unclear whether the use of dating apps like Tinder contributed to the spike, although public health experts have claimed dating app usage has contributed to an increase in casual sex, leaving young lovers at a greater risk of getting infected with STDs.