A leech/maggot-like insect with pointy teeth.
One to four Blue Flies randomly spawn every time Isaac gets hit. No matter how many flies Isaac spawns, no more than 5 will follow him into the next room.
- Fish Head - Stacks, resulting in many more attack flies spawning.
- Chemical Peel - The attack flies spawned by this item will deal increased damage if the last shot fired was a Chemical Peel shot.
Sometimes a fly will attract to a non-enemy entity such as a chest. The cause of this bug is unknown.