In its first ever global assessment of the prevalence of the herpes virus that causes cold sores, the World Health Organization reports in the journal PLoS ONE that two in three humans under the age of 50 are infected with the incurable virus. That's an estimated 3.7 billion people. The WHO had previously done the same assessment for herpes simplex virus type 2, aka genital herpes, and found that 417 million people ages 17-49 have it. HSV-1, by contrast, is transmitted by oral-oral contact (often via childhood kisses, notes NBC News) and causes cold sores. "However, HSV-1 is an increasing cause of genital infection," they write, with the prevalence of genital HSV-1 being highest in the Americas.
Reuters explains why: "Improved hygiene in rich countries" is reducing the number of infections that occur during childhood, and pushing more into young adulthood, where HSV-1 is transferred via oral sex (so not oral-oral but oral-genital). "We really need to accelerate the development of vaccines against herpes simplex virus," one WHO medical officer says. There are currently multiple phase-1 and phase-2 trials, with Genocea Biosciences halting its work on a pneumonia vaccine to focus on genital herpes. As for HSV-1, the WHO estimates that the Americas have the lowest infection rates: 49% of women and 39% of men. That number jumps to nearly 60% in Asia and a whopping 87% in Africa, reports NBC News. (This newborn almost died when a hospital visitor with HSV-1 kissed her.)