Oct. 1, 2008 -- More than half a billion people are infected with the herpes simplex virus type 2, the virus that causes most cases of genital herpes, and nearly 24 million new cases happen per year.

That's according to the first global estimate of the prevalence (total number of cases) and incidence (number of new cases) of herpes simplex virus type 2 (HSV-2) infection.

Here are the estimates, published in the Bulletin of the World Health Organization and based on studies from around the world published in 2003:

536 million people aged 15-49 are infected with herpes simplex type 2 virus. That's 16% of people in that age range.

Every year, 23.6 million people aged 15-49 become infected with the herpes simplex type 2 virus.

The estimates only focus on HSV-2 among people aged 15-49; data came from studies published in 2003. Genital herpes can also be caused by herpes simplex virus type 1 (HSV-1), but HSV-1 typically infects the mouth and HSV-2 typically infects the genitals. HSV-2 is usually spread through sex; it can also pass from mother to child during birth.

Most people with herpes simplex virus type 2 don't know they're infected and don't have symptoms, note the researchers, who work at Imperial College London and the World Health Organization (WHO).

HSV-2 prevalence was higher among women than men and varied across world regions. Western European men had the lowest prevalence rate (13%) and women in sub-Saharan Africa had the highest prevalence rate (70%).

The researchers caution that their estimates "should not be taken as being definitive," partly because some world regions had few HSV-2 studies to analyze.