Two-thirds of the world's population has the virus that causes cold sores, the World Health Organization said Wednesday.
That's 3.7 billion people under the age of 50 with the pesky and incurable herpes virus.
But it causes more than cold sores.
The herpes simplex 1 virus (HSV-1) can also cause sores on the genitals — and oral sex is becoming a leading way it's being transmitted, the WHO reported in the Public Library of Science journal PLoS ONE.
"The global burden of HSV-1 infection is huge," the WHO research team writes.
"An estimated 140 million people aged 15-49 years were calculated to have prevalent genital HSV-1 infection globally in 2012," they wrote.
That means two kinds of incurable herpes viruses are causing sexually transmitted infections in the populations.
HSV-2 is traditionally called genital herpes, and it's the kind most people think of as causing sexually transmitted infections. HSV-1, while annoying and sometimes painful, is usually caught in childhood and often via kisses.
"The new estimates highlight the crucial need for countries to improve data collection for both HSV types and sexually transmitted infections in general," said Dr. Marleen Temmerman, director of WHO's Department of Reproductive Health and Research.
In the Americas, WHO estimates that 49 percent of women, or 178 million women, have HSV-1 and 39 percent of men, or 142 million, do.
That's the region with the lowest rates. In Africa, 87 percent of people have HSV-1 while it's close to 60 percent in Southeast Asia.
Both viruses cause painful and recurring blisters and both can spread even if someone doesn't have skin blisters. Both can be treated with the same antiviral drugs but the drugs just control the virus - they cannot cure an infection.
At least once scientific study suggests that humans caught herpes viruses from chimpanzees.
This article first appeared on NBCNews.com.