67% of the people on Earth have this 'incurable' viral infection
The researchers behind the study write that this is the first time the global burden of this virus has been measured.
And the numbers are a real cause for concern, they say, because the most common way the virus is transmitted is changing in many parts of the world.
HSV-1 is the virus behind what we normally think of as oral herpes, responsible for cold sores. Type-2 herpes simplex virus (HSV-2), meanwhile, is the one we normally consider a sexually transmitted infection.
Most people with a herpes infection have the virus their whole lives, since there's no known cure, although they don't always show symptoms.
The growing problem, however, comes from the fact that HSV-1 can be sexually transmitted and cause genital herpes too.
The unknown risks of a common virus
One of the biggest issues is that public health efforts have focused primarily on preventing HSV-2 infections, the researchers write, not HSV-1 infections. But the latter can actually pose more serious risks than just cold sores.
What's more, according to the new report, is that sexually transmitted HSV-1 is on the rise, especially in the Americas, Europe, and the Western Pacific.
Aside from those cold sores, HSV-1 is the most common cause of sporadic encephalitis (inflammation of the brain). And while this brain infection is very rare, it kills more than 50% of those infected if left untreated, according to the National Institutes of Health.
The growing prevalence of genital HSV-1 is more worrisome in cases associated with pregnancy. Studies have shown that genital HSV-1 (which is highly infectious) is more likely to pass the herpes virus on to a newborn than HSV-2, causing neonatal herpes, which the researchers describe as a "rare but devastating illness with high morbidity and mortality."
It's worth stressing that neither of those serious conditions are common, and in the case of a pregnancy, medication and a C-section can prevent the virus from being passed on.
There's also a chance that a genital (as opposed to oral) HSV-1 infection could increase the risk of an HIV infection, though that has yet to be convincingly shown. Such infections can cause complications for people with weakened immune systems.
Why herpes is changing and why it matters
Paradoxically, the reason that genital herpes from type 1 of the virus is on the rise is that HSV-1 infection rates in the Americas and Europe are declining.
In Asia and Africa, most people who get HSV-1 get an oral infection as children - well before they become sexually active - and the first site of infection seems to prevent infections at different sites, the researchers write. So if you get oral herpes as a kid, you're less likely to get an HSV-1 genital infection as an adult.
That means that in places where HSV-1 infections are becoming less common, adults have a greater risk of getting sexually infected, the researchers write. Women are more vulnerable to the infections than men.
Around the world, more than 500 million people have genital herpes from one of the two virus types. In the US, more than 15% of people have a sexually transmitted herpes infection, according to the CDC.
Still, the changing type of transmission and the overall prevalence of HSV-1 means that prevention and control efforts, which have traditionally focused on HSV-2, should address HSV-1 as well in the future. The same goes for the development of treatment or vaccines; as of today, there are no cures, only drugs to suppress an infection.
"These infections are lifelong and incurable, and many people don't know that they're infected," Dr. Sami Gottlieb, one of the WHO researchers and study co-authors, told Stat's Megan Thielking.
"We need new prevention strategies, and an effective new vaccine or microbicides," he said. "That's a major priority."