Parasitic expansion.
The Flood can survive and grow in almost any environment, but it thrives in a surprisingly narrow band of habitability. The interior of Flood hives provide a controlled environment for accelerated growth, but in time an entire planet's biosphere can be altered to approximate these ideal conditions.
Blightlands are the first step of this coversion process: areas around established hives filled with specialized Flood growths which harvest the vitality of the soil, soak up solar energy, and consume all native life within its boundaries. While they briefly flower with a grotesque vitality, ultimately these zones are themselves consumed to provide raw materials for city-sized spore towers and colossal stacks of strange Flood organs that finish the planet's ruination.