The parasite does not build what it can instead repurpose. UNSC and Banished bases within High Charity's blightlands have been converted into hives of the Flood. They are a hideous sight that barely resemble the structures they once were. There are no survivors or salvage worth recovering in these hideous warrens. These facilities' defenses are also not under Flood control.
Cocooned Bases are a major threat and priority target, but are surrounded by blightland and located deep in Flood territory. They continually spawn Flood units until eliminated, and even when the base is destroyed there are hordes of twisted creatures who will pour out the wreckage. Regardless of the risks, the only way to deal with Flood-infested bases is to burn them to the ground, preferably from orbit. The base slot of the Cocooned Base is freed when the Flood buildings are razed.