Non-sentient animals or corpses not harnessed for combat forms are seeded with Flood cells and transformed into infection form-filled sacs. These diseased growths are one of the most pernicious aspects of the Flood, and a single spore could potentially transform an entire planet's biosphere into quivering masses of diseased mucus and bile if left unchecked.
Blisters "hatch" infection forms when disturbed. Destroying them at a distance is almost always the best option.