The idea of getting ill onboard a regular plane isn't appealing. There might not be a doctor on board, and even if there is, there isn't usually anywhere you can go for treatment. The builders of Air Force One have thought about this problem, and provided for it.
The plane has a mini-hospital on board. It includes all the tools that would be required for emergency surgery. It even has a supply of the President's blood type should it ever be required.