United Nations health experts has said two-thirds of the world's population under 50 have the type of herpes which causes cold sores.
The World Health Organisation has estimated the global prevalence of the disease for the first time.
People usually catch it in childhood.
More than 3.7 billion people under 50 are said to be infected.
Both types of the herpes, HSV1 and HSV2, are highly infectious and incurable.
HSV 1 is primarily transmitted by oral to oral contact and can cause cold sores and HSV2 is entirely sexually transmitted.