What makes the flood and chief op?
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Master chief has luck and started training when he was 8-12 yrs old and was a military genius. He then got the Spartan v1 suit and survived in halo wars(correct me if I'm wrong) and then got cortana in halo 1 along with the Spartan v2 suit. Then in halo 2 he works with the elite (forgot his name) and learned of the plans for the covenant along with the info in halo 1 and 3 for the robot alien people( forgot their name too). As for the flood they evolved and are just parasites that can take control over their host kind of like in the last of us.