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Spaceflight Found To Present A Pesky Complication For Astronauts—Herpes.
Rick Partington / EyeEm / Getty Images

According to the World Health Organization (WHO), it is estimated that the type of herpes simplex virus that causes cold sores (HSV-1) affects up to 67% of the world's population.

For most people, this means the annoyance of an occasional cold sore but nothing too severe. The virus remains inactive most of the time, not causing any symptoms at all.

For astronauts, though, the combination of cosmic radiation, microgravity and the stress of spaceflight have caused the virus to reactivate and even begin causing symptoms in some.

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