Flood blister

Flood growth pods are large, bulbous Flood growths that form on walls and floors, and particularly on surfaces covered with Flood biomatter. They are only seen during advanced stages of Flood infestation, when a Gravemind has formed and consumed enough flesh to support Pure Forms. They have no means of defense, but if ruptured, they explode and release their infectious spores and their Infection Forms.

Appearances
In Halo 3, growth pods were encountered in the Campaign levels Floodgate and Cortana, and in the multiplayer maps Isolation and Cold Storage, though the multiplayer pods do not release Infection Forms.

In Halo Wars, the Flood Growth Pods are called Flood Eggs. They serve as double-edged swords; shooting these pods will sometimes releases precious resources but other times, they will release either a swarm of Infection Forms or a Flood Spore cloud, both of which can destroy infantry squads in seconds.

Trivia

 * Hitting the pods with flame-based weaponry will cause the pod to burst, but not release Infection Forms. It could be possible that the Infection Forms that are supposed to emerge burst along with the pod but it is so quick that it looks like one simultaneous explosion.
 * On Cold Storage, it can be seen that they adhere to surfaces using suction-cup-like appendages.
 * If a pod carries enough Infection Forms, then each individual flood can be seen on your Motion tracker.

List of appearances

 * Halo 2
 * Halo 3
 * Halo Wars
 * Halo Legends
 * Origins