Riding into space on the top of a rocket is something that very few people ever get the opportunity to do. It’s a dangerous journey, but the payoff is something that those of us who spend our entire lives here on solid ground can’t possibly comprehend. It also, apparently, makes your herpes flare up.

In one of the more bizarre but nonetheless interesting reports to come out of NASA this year, researchers reveal that over half of astronauts who have spent time in space have experience herpes flare-ups with varying degrees of severity.

The research, which was published in Frontiers in Microbiology, utilized biological samples collected from the astronauts before their space missions, while in space, and then again after their returns to Earth. As for what actually causes herpes viruses to reactivate in astronauts who had some form of it in the past, the researchers have a few ideas.

“During spaceflight there is a rise in secretion of stress hormones like cortisol and adrenaline, which are known to suppress the immune system,” senior author Dr. Satish K. Mehta of NASA said in a statement. “In keeping with this, we find that astronaut’s immune cells—particularly those that normally suppress and eliminate viruses—become less effective during spaceflight and sometimes for up to 60 days after.”

There are plenty of factors at play here, and the researchers suggest that dramatic lifestyle changes while embarking on space missions — warped day/night cycles, social isolation, etc — ramp up astronauts’ stress levels and suppress immune response. This, in turn, opens a window for the virus to reactivate.

Of the dozens of astronauts found to have herpes virus flare-ups from their space trips, only six of them actually experienced symptoms. Viral reactivation occurred in the others but did not produce symptoms reported by the astronauts.

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