Visually strikingly similar to "Headcrabs" from the Valve video game series Half-Life, these Swarms will rush to a corpse and burrow into it instead. The Swarm allow for multiple bodies to be infected simultaneously to cause a quick wave of Necromorphs that can overpower the player quickly if not dealt with fast enough. The Swarm relies on dead human bodies to attack the player; they will burrow into the corpse and take it over. If the reanimated body is destroyed, the Swarm will rush over to the next available corpse and re-animate said body until killed. The Swarm will quickly insert themselves into a body if not stopped. Quickly dismember the body before the Swarm gets to it, or destroy them before they can reanimate a body for better breathing room.
Swarm seldom attack alone. If there is one Swarm, there may be others. It is recommended to eliminate the Swarm before it is capable of reanimating or infecting a corpse. They will dismember a corpse's head and then insert themselves into the body via the head. They should be dismembered to incapacitate or kill them. As with all Necromorphs (with the exception of Regenerators), escaping from the Swarm will usually prove futile, as they will quickly find a body and chase after you, if they have not claimed one already.
As with the Infector, dismembering any corpses in the room will prevent the Swarm from reanimating them, or will result in the production of weaker Necromorphs.
Sometimes you can predict Swarm attacks as you may encounter a group of corpses on the ground that cannot be moved by Telekinesis, similar to when other Necromorphs play dead.
The death scene is the second one in which Isaac turns into a necromorph, the first being the Divider head component.
If a Swarm kills Isaac or Carver in an area with no air, they will pull their helmet off even though this would kill them, however, the death scene still plays as normal.
Like the Ubermorph, the Swarm bears little resemblance to other necromorphs; It's skin looks much smoother and more alien than the other obviously human based necromorphs. A possible explanation for this is that they are created directly from Corruption. This can be deducted from a seemingly limitless supply of Swarm that a Guardian can create.
The Swarm seem to be like octopuses, being able to squeeze their bodies and make themselves smaller, silping into cracks and holes. Because of the size of the creature, Isaac would of mostly choke to death than rather it going into him and killing him from the inside out. This is improbable, however, due to the extreme amount of blood that Isaac/Carver expel during the death-scene.
It takes only a single Swarm to infect a corpse, dead or alive.
The only Necromorphs produced by Swarm in-game are Wasters and Slashers. but the creation of Wasters is more common.
Sometimes, even after completely dismembering a body, it will not stop the Swarm from infecting corpses; it will simply cause the corpse to generate new limbs.
The Swarm can, much like the Swarmer, attach itself to Isaac and eat away at his health. If Isaac's health runs out while they are attached to him, they will crawl all over him, one eventually finding its way underneath his helmet. Isaac tears his helmet off to fight off the creatures, until one enters his mouth and goes inside his body. Isaac coughs a bit, confused, before falling to his knees and vomits a massive amount of blood. Then, with a sort of growl, Isaac stands up again, his eyes and mouth foaming (almost similar to a Waster), now having become a Necromorph and lumbers off.