I'll remain a soldier until the war is won.
"Using genetic engineering, a third of the human population was altered to carry a set of specially designed genes. This group of humans was then allowed to become infected by the Flood. Once the genes were exposed to Flood DNA, they triggered a cascade of cell destruction that spread throughout all Flood biomatter. The majority of the Flood died off as a result, although they would return in full force ten thousand years later."
~Halopedian