From the article by Jeff Kluger at Time Magazine:

Everything’s different in zero-g

You may or may not want to go to space, but here’s something certain: you definitely don’t want to get sick there. Ask the crew of Apollo 7, the 1960s mission in which the commander contracted a cold, spread it to the other two astronauts and all three of them spent the entire mission trapped inside a cramped spacecraft, sneezing, hacking and griping at the ground.

And that was just 11 days in Earth orbit. What about a year aboard the International Space Station (ISS)? What about a two-and-a-half-year mission to Mars?

 

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