After Kes leaves the series Tom Paris, a pilot, is selected to act as a part time nurse in the sick bay.
In-universe, the reason given is that he studied some biochemistry at Starfleet Academy. However, there are plenty of blue shirts still on the ship who, with a science background – including Ensign Wildman who was a xenobiologist – would be better suited for the role. Indeed, on more than one occasion a couple of blue shirts are seen acting as field medics and triage nurses anyway.
I’m not even sure it makes sense out-of-universe. Kes was given the role to give the character something to do. Paris is already chief helmsman and a bridge officer so didn’t need the extra screen time. If it was to avoid casting a new main character then I don’t recall any episode where the role was so vital to the plot it couldn’t have been fulfilled by a non-speaking extra.