Shocking, isn't it? The body inside--including the gums--is filled with network of blood vessels, which restlessly send invader-fighting cells ; molecules.
So when a fly lays eggs ; they hatch out (in fact, most eggs won't even hatch), the body can effectively fight to kill. However, with gum disease
, when a piece of the gum is dead, no blood is there to fight. Eggs hatch, and come out maggots.