We don’t talk much about chlamydia, gonorrhea, or syphilis, in part because it can seem like they’re not big health issues anymore. But it turns out more and more Americans may be quietly suffering from these once nearly eliminated STDs.

According to a new report from the Centers for Disease Control and Prevention, there were more than 2 million cases of chlamydia, gonorrhea, and syphilis reported in the United States in 2016 — the highest cumulative number ever recorded.

The leap in cases in just one year is truly eye-popping. Between 2015 and 2016, the rate of:

Gonorrhea increased by 18.5 percent to 468,514 cases
Syphilis increased by 17.6 percent to 27,814 cases
Congenital (spread from mother to infant) syphilis increased by 27.6 percent to 628 cases
Chlamydia increased by 4.7 percent, to 1.59 million cases

To appreciate just how astonishing the trends are, consider that as recently as a decade ago, these STDs were at historic lows or near elimination, with more and better screening and diagnostics to help identify cases and get people into treatment.

So what’s behind the spread of these diseases here? There’s no single explanation. Like most health trends, it’s complicated. But here are a few ideas, according to experts:

There’s been a rise in condomless sex among men who have sex with men
STDs are spreading more broadly and into populations that weren’t traditionally affected — like babies
With the rise of dating apps, sex is more readily available and more anonymous — and that makes it harder for health investigators to track outbreaks
The numbers may be higher because we may be better at detecting cases in some groups
Cuts to public health funding mean fewer STD clinics