Imagine this, Vermin infest your food stocks. Unwitting dwarf eats said food stock, becomes sick, and dies. Becomes zombie, or worse, hundreds of vermin sprout from Urist McVictim's body. Or worse, a bite from say, an antman (Kudos to wannazzaki), or some wierd ass cavern beasty causing babies to sprout from the victim or WORSE, transform the victim into that same creature... Kinda like a were beast but more gruesome.
There could even be forgotten vermin of a sorts....
THE LIST OF POSSIBILITIES IS ENDLESS (And disgusting)
It would mean you would have to quarantine the crap out of infected dwarfs, have to make better food stockpiles than simply laying them around cave floors and actually having to use... Armok forbid.... Cats.
Or just make cavern critters more !!FUN!!.
Lets be fair, turning your colleseum pit into an infernal parasitic humanoid breeding ground where you can throw goblins, unwanted migrants and the hapless now paralyzed and infected forgotten beast to birth bearded, hugenicegoblin and FB-antmen hybrids would give us something new to inflict upon the world.
IT'S TOO MUCH POWER FOR ONE DWARF TO WIELD
...
So Toady, do give it do everyone. pretty please
Could be a mod in it's own right. DF Defence: Attack of the Stingwing Antmen