A lot of people are infected with the herpes simplex virus. According to the World Health Organization, an estimated 3.7 billion people under the age of 50 (or 67%) are infected with HSV-1, globally. HSV-2 is less common, but still prevalent. "If you were to give everyone blood tests, the number would be very high — up to 80% of people have HSV-1 — but a lot of people who have HSV-1 don't realize it because they never have any symptoms," Green says.
Most oral and genital herpes infections don’t cause any symptoms at all, according to the WHO. "The virus can lay dormant in dorsal root ganglia, a cluster of nerves on the spine," Green says. So you can live your whole life with HSV-1 and never get a cold sore. "Cold sores usually appear when the immune system is stressed — so when you're sick, fighting another virus or infection, have a debilitating cancer, or fatigued."
That’s actually why they’re called “cold sores,” Green says, because they tend to pop up when you have a cold. But the cold virus does not cause the sore; instead the body is more vulnerable, which can lead to a reactivation of herpes virus that is already present in the nervous system. Cold sores can also pop up if the body is stressed, like after surgery or too much sun exposure.