Sadako said:
I've got a bit of headcanon that states that midi-chlorians are attracted to the Force and sustain themselves on the Force, and so beings which are Force sensitive can sustain larger colonies of midi-chlorians, but that the connection ends there.
I go a bit further myself and run with the idea that they're bonafide Force-sucking parasites which rewrite the DNA of their hosts to make them dependant on their presence to survive.