Sorry, I'll reword it.
So I am assuming that most people that become infected are infected at a single point in time or at least in a short period of time by a single person or handful of people :- They will likely be infected by the virus in a particular point in its evolutionary life cycle and hopefully overcome it.
Front line medics on the other hand are likely to come into contact with literally thousands of infected people over a much longer time frame :- They are likely to see greater variety in the virus as it mutates from person to person, in some cases it might be more harmless and in others it could have mutated in a particularly nasty way or in a way that has evolved to overcome the natural immunity the medic has developed.
Is this logical/biologically viable? Essentially does immunity generally protect from the virus when in place or can a virus mutate in such a way/rate that continual exposure over a length of time to a wide variety of carriers increase the risk of a critical infection?