Zero gravity and bodily fluids are a dangerous combination.
If humans are to one day explore beyond our moon -- to Mars, for example -- there are some pretty huge logistical hurdles that will need to be overcome. How do you prepare and package food that will last for a two-year mission? What do you do with the waste humans create?
And here's another: What happens if one of the humans has a medical emergency and needs surgery? As Carnegie Mellon professor James Antaki told New Scientist, "Based on statistical probability, there is a high likelihood of trauma or a medical emergency on a deep space mission."
This is not just a matter of whether you'll have the expertise on board to carry out such a task. Surgery in zero gravity presents its own set of potentially deadly complications.
Think about how hard it is to pee in zero gravity: You need a funnel and a tube that siphons your urine to a sewage tank. Without those tools: Urine everywhere. Consider the difficulties of brushing your teeth. It took astronaut Leroy Chiao three paragraphs to explain that process, and it involved bungee cords, drink bags, and velcro.