Comment SSBNs as an analogy (Score 1) 70
The US Navy's Fleet Ballistic Submarines (SSBNs) will stay at sea and submerged for months at a time. It's similar to a Mars mission: For large parts of that, they've out of high-bandwidth communications 24x7, and that can't be relaxed even if somebody gets ill. The populations are young and fit, but medical emergencies do happen, particularly injuries.
SSBNs do have excellent medical facilities, but no physician. One or sometimes two Submarine Independent Duty Corpsmen handle medical emergencies, if need be without help. The training for a Submarine IDC used to be year; I think it's been or is being upgraded.
And people go on those missions voluntarily.