The idea being that if a pony has too much magic for their body it starts breaking the cells down. in light cases as long as the pony uses magic there is little problem as the body has the breathing room to repair. ponies suffering from magic sickness also can’t be heal be magic as their cells are already to full to take in the magics healing.
Gear as a genetic version of this sickness, her body producing crazy amounts of magic. She figured out that well others healing magic your own does, so she became a doctor out of necessity. it’s not enough for her to be health, but it had made it so Gear is more annoyed at things like her organs failing then anything.
She’s a strong willed and hand working, using her power healing magic to cure other ponies.
She also likes sweaters and usually wears them