Bob is a patient of Markiplier's, who strangely has to be operated numerous times, such as having heart transplants or even brain transplants.
Rival with MarkEdit
It is implied that Bob "ruined" Mark in Nicaragua, possibly meaning that they had some kind of rivalry. Bob was probably picked first at medical recess, as Mark remembered not being picked for the dodgeball team while complaining about Bob. Bob also beat in something, said something unintelligible due to Mark's mindless screaming. It could be made out as "KILLINGS TOGETHER!". 
This could explain why Mark is so sadistic, cruel, and hostile to Bob.
- ↑ Surgeon Simulator 2013 | Part 1 | I'M A DOCTOR!!