Infected wounds.
Spore Mounds are an agglomeration of lesser Flood forms and secretions that conceal and protect the foul nests of the parasite. Though mere hills compared to what is to come, the Spore Mounds which have sprouted on the Ark and survived Sentinel cleansing efforts are incredibly dangerous.
Spore Mounds act as an Abomination leader for nearby Flood, in addition to producing new units.