Forerunner economics

But another, far greater concern quickly came to the fore: the Flood. Our earliest encounters with that shape-changing and all-consuming plague had been shocking. The Flood ripped through hundreds of Forerunner battle fleets and dissolved their crews into crawling, agonized muck, or grouped them into amazing collectives we called Graveminds. Warrior-Servants methodically destroyed the infected fleets, leaving only scattered remains to analyze—damaged monitors and broken bits of armor.
(Halo:Silentium; Ch.3)