More than 3.7 billion people under the age of 50 -- about 67 percent of the global population -- are infected with herpes simplex virus type 1 (HSV-1), according to a new report from the World Health Organization.

Herpes simplex virus is categorized into 2 types, both of which are highly infectious and incurable. HSV-1 causes lesions on the lips or around the mouth commonly referred to as cold sores or fever blisters. It's primarily transmitted by oral-to-oral contact.

"Most people contract the virus when they're kids," CBS News medical contributor Dr. Holly Phillips said. "It usually lies dormant in the system. It's incurable. You never get rid of it but it lies dormant most of the time. Every now and again people have outbreaks and that's when you can see the blisters."

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Phillips explained that what sparks outbreaks varies from person to person, but there are some known triggers. "For most people, when their immune system is under stress is when you start to see the blisters," she said. "If they have an illness like the cold or flu -- that's actually how they ended up being called 'cold sores' -- or if you're just fatigued. Some people, if they get a lot of sun exposure, they'll get an outbreak, or women during pregnancy or when they have their menstrual periods. Really anything when your immune system is under attack, these cold sores will come out."

The other type, herpes simplex virus type 2 (HSV-2), is almost entirely sexually transmitted through skin-to-skin contact, causing genital herpes. But the WHO's findings, which were published Thursday in the journal PLOS ONE, highlight that HSV-1 is also an important cause of genital herpes.

"It's important to point out that both types of herpes can cause sores in both areas, but the World Health Organization in this report, they did emphasize that there is an increase in the number of genital herpes actually now caused by type 1," Phillips said.

HSV-2 can lead to more complex health issues, including swelling in the brain and increased risk of spreading HIV.

Experts say the numbers in the new report highlight the need for better education regarding how the virus is spread.

"Access to education and information on both types of herpes and sexually transmitted infections is critical to protect young people's health before they become sexually active," Dr. Marleen Temmerman, Director of WHO's Department of Reproductive Health and Research, said in a statement.