As you can imagine, medicine in space is completely different than it is here on earth. Our bodies operate in many different in zero gravity. One would not want to get sick in space. Zero-gravity plays a lot of games with our bones, muscles, organs, eyeballs and the brain itself. There's also infectious risks on a spacecraft which stems from sealing multiple people inside a self-contained vessel. Virus or bacteria could simply circulate around from person to person throughout an entire mission.