Boil

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.

Boils are nearly harmless on their own, as they can simply be killed from a distance and do not deal contact damage. 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, especially when fighting Gurdy.

Gut
Guts can replace Boils or Sacks. Instead of firing disorganized blood shots, Guts fire shots at Isaac that do not damage them.

They can easily be a nuisance if other enemies are backing them. However, they are rather easy to manipulate into destroying rocks or finding Secret Rooms without using a bomb.

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, , or the.

Gush
Gushes are enemies similar in appearance to Black Boils. They are stationary, being unable to do any form of damage. Instead, they continuously create slowing Black Creep around them. Unlike other Boil variants, they do not shrink or regenerate. They also do not need to be killed to complete a room and will stop spewing creep once all other monsters are killed.

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, 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 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.

Trivia

 * A pustule is the scientific name for a pimple.