Flood growth pods

Flood Growth Pods are large, spherical collections of biomass that grow on the walls of ships and buildings. 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.

They are only seen in Halo 3 in the campaign levels Floodgate and The Covenant, and in the multiplayer map Isolation.

Trivia

 * It was originally thought that they spewed Infection Forms if shot at in Isolation, however, this was proved to be only a rumor. Instead, they let out a shriek like they do if damaged during campaign mode.