Flood blister

Flood growth pods are large, spherical collections of biomass that grow on the surfaces of ships, buildings, and other structures. They are started when mass Flood forms enter an area, or when Flood Spores enter the area. They incubate developing Infection Forms. They are only seen during stages of advanced Flood infestation when a Gravemind has formed and consumed enough flesh, to support Pure Forms. They have no forms of attacks, but if they are attacked, they explode, and release their infectious spores and their Infection Forms.

They are first seen in Halo 2, during the Flood Invasion of High Charity, though it was only at development stage. Later, in Halo 3, it was encountered more 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.

The use of Flood Growth Pods, is quite similar to Carrier Form Flood; they incubate the Infection Forms for a certain time and if the pods are agitated (i.e. being shot at or hit) they burst open and spew Infection Forms (between five and 20 Infection Forms). Unlike the Carrier, however, the pods are completely idle and mostly defenseless. They can't suicide bomb the player like a Carrier can, but still pose a threat due to the large number of Infection Forms they release, causing players to think twice before they begin spewing rounds around a room to try and hit anything that moves.

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 Sensor. Try to stay away from those pods.