The potential use for this in a monster manual is huge, albeit a bit appalling and it does scream GM fiat all over. Imagine all those little critters you once found in a dungeon now laying eggs on mosquitoes that fly into town. Remember the dreaded rot grubs? Now you don't have to journey deep into the forest or a dungeon. They're delivered fresh to your home by your friendly mosquito.
Or maybe the villages honey bees are now infected with some terrible bug using them as vector insects.
Why stop with insects. Curses and even zombification can be airborne on mosquitoes and other vector insects. Maybe even the G'ould evolved to be airborne that way. Chilling thought.