Hidden Flea Area:
Onn the Lower East Quack level, travel beyond the area where you catch a ride on the subway train. Go to the end where you see a brown fence and a little hole near the lower left side. When you are there, turn left to see a Chang transport. Once you are on Chang go though the hole, and on to the boxes. Chang will start scratching himself all over. You will then be transported on his back as a flea with a heavy-machine gun.

Thanks to Revolution readers Nathan Jon Fernandez and Grimlok!