The flood

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.^[1] They have no forms of attacks, but if they are attacked, they explode, and release their infectious spores and their Infection Forms. They tend to release about 20 Infection Forms if shot at or if hit by the blast from an explosion in their immediate vicinity.^[2]

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.^[3]

The Colony Form is a Halo Wars Flood structure.^[1] They are seen in more heavily infested areas of Flood, and are attached to the Proto-Gravemind. This one becomes weaker when a Colony is destroyed.I have got this info from halopedia.com