When the Precursors unleashed the Flood on the prehistoric humans, they passed a test and the Flood chose not to infect them, according to the Primordial. What is this test and why did the Forerunners not pass it?
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The only known cure turned out to be Johnson's unique resistance to the Flood, caused by his supposed Borens syndrome. And, perhaps, some minor augmentations from the S1 program.