Parasitoid

Effects

Adds a chance to fire an egg sack which spawns slowing creep. If an egg sack hits an enemy, one to two Blue Flies or Blue Spider is spawned.
Egg sack tears have considerably higher tear height and consequently, range.
(Added in Repentance) This item belongs to the Beelzebub set. Collecting three items from this set will transform Isaac into a giant humanoid fly.
(Added in Repentance) This item belongs to the Spider Baby set. Collecting three items from this set will transform Isaac into a humanoid spider.

Egg sacks will also spawn blue flies or spiders if they hit Fireplaces or invincible enemies like Death's Head and Stoney.
The chance to fire an egg sack is affected by luck. It is 15% at base luck (0), but cannot reach 100% at high luck.
It is possible to create a large number of Spiders and Flies by repeatedly shooting a closed Host.

Compound Fracture / Cricket's Body / The Parasite: When egg sacks are fired, they will split into smaller egg sacks. Split egg sacks from The Parasite will not look like egg sacks.
Cupid's Arrow: Egg sacks will apply their effect to every enemy hit.
Hive Mind: Flies and spiders are bigger and deal double damage.

(Azazel) Azazel: Overrides Parasitoid.
Brimstone: Overrides Parasitoid.
Dr. Fetus: Overrides Parasitoid.
Epic Fetus: Overrides Parasitoid.
(Fish Tail) Fish Tail: No effect.
Haemolacria: Tears retain Haemolacria appearance and only the main tear can spawn flies or spiders when damaging enemies.
Ipecac: Only extra range applies on egg sack tears and they will not spawn flies or spiders.
Mom's Knife: Overrides Parasitoid.
(Added in Repentance) Montezuma's Revenge: No effect.
(Added in Repentance) Revelation: No effect.
(Added in Repentance) Spirit Sword: The sword projectiles can still spawn flies and spiders, but the swings themselves won't.
Tech X: Overrides Parasitoid.
Technology: Overrides Parasitoid.
Trisagion: Overrides Parasitoid.