Boils start as small red spheres that slowly grow to full size. When at their full size, they can spurt out a cluster of 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.
Variants
Gut
Guts can replace Boils or Sacks. Instead of firing disorganized blood shots, Guts fire 
Ipecac shots at Isaac that do not damage them.
Sack
Sacks don't shoot projectiles; instead, they periodically spawn a Spider when fully swollen. Therefore, damaging them will prevent them from spawning Spiders, making them easy to kill. They also regenerate much slower than Boils or Guts. They can commonly be found on the early floors.
Blue Boil
Blue Boils are mainly spawned during the Hush fight. They spawn fully swollen and don't need to charge up like regular Boils. Instead of spraying clusters of blood shots, they fire either one blood shot, or three blood shots in a spread towards Isaac. They also regain their health 10x faster than regular Boils if they are damaged but not killed.
They can also rarely appear in 
Depths, 
Necropolis, or the 
Dank Depths.
Notes
- Blue Boils that appear in regular rooms can be Champions.
- Blue Boils' shots deal a full heart of damage to Isaac.
They can also appear in ??? with the use of Red Key or by entering Treasure Rooms with the Blue Key.
Gush
Gushes are stationary, being unable to do any form of damage. Instead, they continuously create slowing Black Creep around them that slowly expands infinitely in radius and can cover the entire room if left alive for long enough. Unlike other Boil variants, they do not shrink or regenerate.
Although the Black Creep is relatively harmless on its own, Tar Boys can use their special attack if Isaac is standing on the creep.
Trivia
- Gushes were added in Booster Pack #4, after the initial release of the DLC.
Pustule
Pustules are stationary enemies that appear to grow out of the ground, unlike Boils they do not shrink when damaged and instead continuously swell in size. When destroyed at the smallest stages, they explode into a tiny puddle of Red Creep and don't spawn anything. When destroyed at the last 2 stages of swelling they spawn a Small Maggot and a small pool of Red Creep, and if not destroyed, they will burst into a Bloodfly and a slightly larger puddle of Red Creep.
Walking Boil
Walking Boils act like Boils, but randomly wander around the room, can deal half a heart of contact damage, and don't shrink or regenerate.
Walking Gut
Walking Guts act like Guts, but randomly wander around the room, can deal half a heart of contact damage, and don't shrink and regenerate.
Walking Sack
Walking Sacks act like Sacks, but randomly wander around the room, can deal half a heart of contact damage, and don't shrink or regenerate.
Notes
- While regular Boils can be touched without taking damage, champion forms do inflict one heart of damage on touch (like all champions).
- Champion Boils grow significantly slower but take more hits to deflate. The exception to this is gray champions, which grow faster and take fewer hits to deflate.
While regular Boils can be touched without taking damage, Trivia
- A pustule is the scientific name for a pimple.
- Pustules in Antibirth would burst into Small Maggots as puddles of Green Creep as opposed to the Bloodfly and Red Creep in Repentance.
 The Binding of Isaac: Rebirth
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