The bad news is you probably have herpes. At least it's not the type you’re thinking of

Two-thirds of the world's population under 50 have the highly infectious herpes virus that causes cold sores around the mouth, the World Health Organisation said, in its first estimate of global prevalence of the disease.

More than 3.7 billion people under the age of 50 suffer from the herpes simplex virus type 1 (HSV-1), usually after catching it in childhood, according to a the WHO study published on Wednesday.

That is in addition to 417 million people in the 17-49 age range who have the other form of the disease, HSV-2, which causes genital herpes.

HSV-1 normally causes mouth ulcers rather than genital infection, but it is becoming an increasing cause of genital infection too, mainly in rich countries.

That is because improved hygiene in rich countries is lowering HSV-1 infection rates in childhood, leaving young people more at risk of catching it via oral sex when they become sexually active.

HSV-2 can increase the risk of catching and spreading HIV, the disease that causes AIDS. Little is known about any link between HSV-1 and HIV/AIDS, although it can lead to other serious complications such as encephalitis.