Reactivation of herpes viruses is more likely to occur in astronauts the longer they are in space, a new study says.

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NASA researchers checked for eight human herpes viruses in blood, urine and saliva samples from astronauts before, during and after shorter space shuttle flights and longer International Space Station missions, CNN reported.

Four of the herpes viruses — including oral and genital herpes, chickenpox and shingles — were reactivated in more than half of the astronauts, according to the study recently published in the journal Frontiers in Microbiology.

“To date, 47 out of 89 [53 percent] astronauts on short space shuttle flights, and 14 out of 23 [61 percent] on longer ISS missions shed herpes viruses in their saliva or urine samples,” said study author Satish Mehta at Johnson Space Center, CNN reported.

“These frequencies — as well as the quantity — of viral shedding are markedly higher than in samples from before or after flight, or from matched healthy controls,” Mehta said.

Shedding refers to reactivation of a virus. Herpes viruses take up residence in nerve and immune cells, so they are never really gone and can “wake up,” CNN reported.

Reactivation of the viruses doesn’t necessarily mean that the symptoms return. “Only six astronauts developed any symptoms due to viral reactivation,” Mehta said. “All were minor.”

However, if viruses reactivate while astronauts are in space, they could infect others when they return to Earth, CNN reported.

The reason why herpes viruses can reactivate while astronauts are in space is the same as on Earth — stress.

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