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.
1. Organs settle in a different position: On Earth, our organs settle into the predictable positions. A doctor would know exactly where they settle in ones body if they needed to access them such as palpating the liver or pumping the chest. In zero gravity, not so much. The organs can be displaced and shift up a little more. The heart may have a different orientation which can be reflected in an EKG. Other kinds of shifting or compression of the lungs, stomach, bladder can cause issues.
2. Bones and space don't mix: Gravity keeps your bones working hard and strong. Without that, the skeleton doesn't work as hard and it can cause the bones to decalcify and weaken. Astronauts spend hours per week exercising to counteract some of these effects.
3. Your eyes hate space: Those who've been in space for a long period of time, often find that their vision can grow worse and it doesn't always bounce completely back, when they return to Earth. This stems from fluid shifting upward from the lower body into the head, which compresses the optic nerve and distorts the shape of the eyeball. Eye infections and irritation are more common in space.
4. Healthy feet help in space: If you suffer from callouses on your feet, this can actually benefit you in space. They serve a purpose and help cushion your foot against the shock of walking but since you're not walking in space, you don't need them.