I imagine astronauts get bored floating around in space. What better way to pass time than to spread diseases? Sharing is caring, y'all.
NASA has issued a report that 61% of their space crews test positive for oral and/or genital herpes. Somehow, the dormant illness can be triggered due to a decreased immune system. There's a lot more science-y stuff about it here. In the meantime, if you're planning a trip into outer space, try not to make out with any aliens.