When you imagine the when and the where of how you might have contracted oral herpes—the kind that causes those painful, hard-to-hide cold sores around your mouth—you probably picture one of three scenarios. A middle-school game of spin the bottle, a horny high-school hookup, or a drunk night out turned one-night stand.
But hang on, consider some new scenarios: A dad kissing his son's feet, two toddlers smooching in preschool, a grandmother giving her granddaughter a raspberry on her cheek.
It sounds innocent enough, family and playmates showing one another affection. But it's common to contract the herpes simplex virus type 1 (HSV-1)—the strain most commonly associated with oral herpes, as opposed to HSV-2, which causes genital herpes—from these seemingly harmless interactions.
Even if you're not having an outbreak, the virus lives in your saliva. "It's typically transmitted just by contact—a kiss from a relative, kids playing in daycare," says Sami Gottlieb, a medical officer at the World Health Organization.
You were probably hanging on monkey bars and playing on seesaws with little herpes-infected children as far back as preschool. You might have been one of them yourself: More than a quarter of kids worldwide under the age of 5 have HSV-1, according to a 2015 study published in the journal PLOS One. In North and South America alone, 20 million kids under the age of 10 have the virus.
And across the globe, 528 million kids under the age of 10 are infected. That's more than half a billion children who have yet to reach a double-digit age group.
To calculate these estimates, researchers gathered data from all existing studies that tested people for HSV-1 antibodies, a substance your body creates in response to the virus. Antibodies for some conditions, like chickenpox, signal that you were at one point in time infected but your body has since fought it off.
"HSV antibodies are a marker for HSV infection, but having them doesn't mean the infection has been cleared or resolved," says Gottlieb, who co-authored the study. "HSV infections are lifelong." Infection rates vary greatly from region to region. About half of the female population and 40 percent of the male population in the Americas, the region with the lowest rates, have the virus. In Africa, the region with rates higher than any other, 87 percent of the total population is infected.
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"In areas with dense populations, more close contact, and less hygienic living conditions, there's thought to be more transmission of the virus," Gottlieb explains. In some ways, that describes an average day in the lives of free-spirited kids—they kiss their friends on the playground, they get kissed by their parents, they stick their hands in or near other kids' mouths, and then in their own. "It can be transmitted very easily," she says.
Most people are basically walking herpes transmitters. Kids—and adults—are much more likely to contract the virus from direct contact with another person than from inanimate objects like drinking glasses or toys that have been covered in spit. "The likelihood that a herpes virus would stay alive on a toy and transmit to someone else is low," says Terri Warren, a registered nurse and adult nurse practitioner who has participated in about 100 studies on sexually transmitted infections, including herpes. "The virus needs human cells, it needs warmth, and it needs moisture to survive."
Parents have a free pass to be bat-shit crazy about protecting their newborn baby up until they're at least six weeks old, says Yvonne Bryson, a professor of pediatric and infectious diseases at the University of California at Los Angeles. "That's when it's most dangerous to a baby." In other words, if you have kids, you might want to conduct a quick visual vetting procedure before you hand anyone your precious cargo. Do you notice any cold sore or ulcer symptoms near their mouths? Then hold on tight—in the early days of a baby's life, an HSV-1 infection can be fatal.
Warren heard of this firsthand from fellow health care providers: A well-intentioned grandfather paid his healthy newborn grandchild a visit. He had a cold sore, but most likely, nobody thought anything of it. He probably hugged, kissed, and held the baby in his arms like any other family member had. Soon after, the baby started having trouble nursing; in this case, it was the first sign of infection. After that, its health deteriorated quickly. The newborn was hospitalized and died about a week later.
"We need to be careful about talking to parents and grandparents about not kissing newborns when they have cold sores, because often the baby has no immunity at all," she says. "They are particularly vulnerable." After those initial six weeks, a baby's immune system builds up and the risk of life-threatening infection drops to almost nil. That's when your crazy pass expires—sure, you can bend over backwards to protect your kid, but two-thirds of the world's population carries HSV-1, and many people with the virus are asymptomatic. In other words, it's nearly impossible to know who is infected out of a crowd unless they have an outbreak.
When the virus enters the body, it travels up the nerves and hangs out in the ganglia, a bundle of nerve cell bodies. There, the virus lies dormant—but intermittently it can travel back down the nerves and surface on your skin or mucus membranes, causing an outbreak. "There's more virus present when you have an active sore like an ulcer, but the virus can be shed even when there aren't any symptoms," Gottlieb explains, so it's difficult to shield yourself or your kid from every threat.
But there is at least one silver lining: Contracting the oral herpes virus as a kid could actually protect you from severe outbreaks in adulthood. "If you have HSV-1 antibodies, you're still just as likely to acquire an HSV-2 infection," Gottlieb says, "but you'll be much less likely to show symptoms of that infection." Read This Next: Everything You Need to Know About Herpes and Sex