Two-thirds of people under 50 have oral herpes, according to the World Health Organization’s first estimate of the virus’s scope. HSV1 affects 3.7 billion people worldwide, while HSV2, which typically causes genital herpes, affects an additional 417 million people aged 17 to 49. WHO officials are calling for an increased effort to find a vaccine, as the the viruses can increase transmission of HIV when they affect the genitals.