Immune

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Finish a co-op mission without becoming infected

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In order to become infected, a Yellow Flood Spore will need to jump on you and latch itself onto your back. You will notice that when this happens, even though you can still shoot your gun and melee, you will not be able to remove the Flood Spore from yourself. After about five to ten seconds, you will immediately die and the Flood Spore will disintegrate unless your teammate shoots it off of you.

With this in mind, you will essentially need to avoid getting hit whatsoever by these Yellow Flood Spores. Just make that these Spores are top priority when they enter your field of view. If you can complete the entire mission without getting hit by on, or one latching onto you, you will unlock the achievement.

The Repel AA is quite useful when going for this achievement because once activated, it dispels a grey mist that repels the Flood for about five seconds. This gives you just enough time to back up, reload if necessary, and pour bullets into the Flood.