Just giving my share for this very nice thread.
I'm not sure if it's the top of it's category but this parasite is definitely disgusting.
The Dragon Worm
The Guinea worm, or Dracunculus, meaning ‘little dragon’ in Latin, is caught by drinking dirty water containing tiny water fleas infected with larvae. Once inside someone, the spaghetti-like female worm grows up to a metre long and emerges through the skin, usually on the legs or feet, causing a burning pain that inspired the parasite’s name. The pain can last for weeks and there’s no vaccine – the only way to get the worm out is to gradually wrap it around a stick.