Cold sores on the lips can be embarrassing and tough to hide. But, turns out, you might not have a reason to be embarrassed.

"About 70-plus percent of the U.S. population has been infected with herpes simplex 1. Now, a very small percentage of those people will actually develop cold sores."

Dr. Pritish Tosh, an infectious diseases specialist at Mayo Clinic, says, genetics determines whether a person will develop cold sores.

"A proportion of the population, they don't quite have the right immunologic genes and things like that and so they're not able to handle the virus as well as other people in the population."

The problem is people can spread the herpes virus whether they develop cold sores or not. Herpes virus spreads through physical contact like kissing, sharing a toothbrush — even sharing a drinking glass — or through sexual contact.

"Since the number of people who are infected but don't have symptoms vastly outnumber the people who are infected and have symptoms, most new transmissions occur from people who have no idea that they are infected."

For the Mayo Clinic News Network, I'm Ian Roth.