Boils are spawned by Gurdy, the green alternate Chub, the pink alternate Widow and also found in The Caves/Catacombs and The Depths/Necropolis.
Behavior
They start off as a small red sphere and slowly grow to full size. When at their full size, they can spurt out short-ranged blood shots at Isaac, similar to one of Monstro's attacks. Boils decrease in size when hit, but will regenerate to their full size until fully destroyed.
Boils are nearly harmless on their own, as they can simply be killed from a distance and do not deal damage on contact. With other enemies, however, they are far more dangerous: If entering a room with a large crowd of enemies and several Boils providing covering fire, taking out enough of the Boils to give yourself breathing room should be a big priority.
Champion Varieties
Gut
Guts are new enemies added in the Wrath of the Lamb. They can replace regular Boils, and appear naturally in Necropolis.
Instead of firing multiple short-ranged blood shots, Guts fire IPECAC shots at Isaac that do not damage themselves.
They can easily be a nuisance if there are other enemies backing them. However, they are rather easy to manipulate into destroying rocks or finding Secret Rooms without using a bomb.
Sack
The third variant was also added in the Wrath of the Lamb, they cannot replace Boils or Guts, but can commonly be found in the Cellar, and can be spawned by Widow.
Sacks don't shoot projectiles, instead, they periodically spawn Spiders when fully swollen. Therefore, damaging them will prevent them from spawning Spiders, making them easy to kill.
Champion Varieties
Walking Variants
The walking variants attack like their normal variants, however they can walk. They are most commonly found in Necropolis. Damaging them will not cause them to decrease in size, but they do not regenerate their health, either.