Due to this suppression of the immune system, the astronauts' bodies are less able to keep dormant viruses down, allowing them to reactivate.
"To date, 47 out of 89 (53%) astronauts on short space shuttle flights, and 14 out of 23 (61%) on longer ISS missions shed herpes viruses in their saliva or urine samples," reported Dr Mehta.
"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."
"Only six astronauts developed any symptoms due to viral reactivation," said Dr Mehta. "All were minor."
NASA issues space herpes warning as virus reactivates in astronauts