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While herpes viruses cause harmful or annoying afflictions like chickenpox and cold sores, there’s also evidence it can help your immune system fight unrelated attackers.

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If you know just one thing about herpesviruses, it's probably that once you get infected by one, you have it for life. Which, obviously, stinks in many ways.

After all, these viruses are behind things like mono, chickenpox and shingles, infectious blindness, and even some cancers. But it turns out that they might also be kind of good for you. Having herpes may protect you from things that have nothing to do with herpes, like the freaking plague.

And that's because herpesvirus infections do something immunologists have only recently realized was even possible: they train your first-line immune defenses! Now, we've known for a very long time that the human immune system can remember pathogens and launch a better counter assault if they dare show up again. That's thanks to the adaptive immune system: the arm of the immune system that includes things like antibodies, those Y-shaped molecules that glom onto viruses and the like, taking them out of commission.

But apparently, your innate immune system can learn from experiences as well. That's the arm of the immune system that spots infectious agents to begin with and takes the first stab at kicking them to the curb. And for more than a century, scientists didn't think it could remember anything, let alone use that memory to adapt how it responds to threats.

I mean, it's right there in the names: innate versus adaptive. In fact, the idea it can remember and adapt is so new that the term for it— “trained immunity”—was only proposed in 2011. The innate immune system doesn't remember specific attackers like the adaptive immune system does.

Basically, it just remembers that bad guys exist and that they could come after you at any time, like they've done before. But this is probably enough to alter how well you fight off a potentially deadly infection. And apparently, one way to get this trained immunity is with a little help from a herpesvirus.

Which is a weirdly constructive way to think about herpes. “Herpesviruses” are members of the Herpesviridae family, eight of which infect humans. And they're everywhere. The odds of you getting at least one in your lifetime is pretty high.

Two-thirds of young people in the world probably have Herpes Simplex 1, for instance, the virus behind cold sores and some cases of genital herpes. And they'll have it forever, because herpesviruses can do something few viruses can. They can hunker down inside cells and remain dormant for long periods of time —a feat known as latency.

And this is where the immune training idea comes in. Studies suggest that while herpesviruses are hanging out in their hosts, they can help the immune system fight off deadlier pathogens. See, while latent infections generally aren't attacked like active ones, they don't go totally unnoticed by your immune system.

So, because they're there, your innate immune system essentially keeps its bouncers on high alert. That high alert state is your trained immunity. And experimental research in mice suggests it could save your life.

In a 2007 study, researchers exposed mice to two different bacteria: Yersinia pestis—aka the plague—and Listeria monocytogenes, which causes very serious foodborne infections. Mice that were latently infected with mouse versions of herpesviruses were resistant to both bacteria. Ah, the perks of having herpes.

These perks didn't start right away, though. Mice that were still in the acute stage of infection—so, before the virus had established latency—weren't resistant to the pathogens. That's all in mice, of course, and scientists haven't definitively proven that infections in humans similarly lead to better defenses against other pathogens.

Because, well, we don't generally infect people with deadly pathogens just to prove their herpes is protecting them. But there are clues that human herpesviruses confer similar benefits. Like, when researchers looked at the immune cells of people with latent infections of the herpesvirus CMV, they saw changes consistent with trained immunity.

Specifically, they found differences in the participants' natural killer cells—immune cells awesomely named because they recognize and kill infected cells. In infected participants, these cells produced more interferon gamma, a protein which helps your immune system to keep an eye out for trouble. And researchers think heightened state of vigilance could last up to a few years after a latent infection starts.

So while herpesviruses hide in our bodies, they may also help those bodies stay alive longer. A frenemy-with-benefits kind of situation. Which, from an evolutionary perspective, makes sense for the virus, too, since it can't replicate inside dead cells.

Now, trained immunity may not always be a good thing. Researchers think that it could go awry and lead to autoimmunity —the immune system attacking a person's own tissues. Still, it's probably pretty useful a lot of the time.

And further research on the phenomenon could really help people. Like, eventually, studying herpes could teach scientists how to give people trained immunity without the viral infection. In fact, some preliminary research suggests certain vaccines are already doing this!

We just need to take a closer look to figure out how. And this could mean we can finally get a jump on some of our more persistent foes. And hey, since most people are stuck with these viruses anyhow, it's nice to know that there are some upsides.

Thanks for watching SciShow! [♪outro].