How does the HUD/VISR differentiate between flood?

Question

I was doing some research on Halopedia and Halo.wikia, and came across how enemies were tagged by VISR and its variants. (UNSC only- not Covenant)

Essentially, all infantry units were, by some means, tagged with a IFF tracker. When they were within a friendly's range, they were marked with a yellow dot on the motion tracker.

All other units though, that did not have a transponder registered to a UNSC frequency and were subsequently marked as enemy.

I remember reading that the IFF tracker was inserted in the back of the neck (I assume around the barcode area you see in the Landfall mini-movie), however I also read that it is set in the equipment.

Which brings me to this question: How does the HUD/VISR differentiate between flood?

Granted lore wise, we know they are enemies. I mean go figure. But as the flood combat forms still use the equipment, and don't tear out the tracker, how does the HUD/VISR differentiate between friend and foe?

I am looking for a more story based than gameplay mechanic based answer, but all replies are welcome. Thank you!

Answer 1

All UNSC personnel are implanted with neural chips or interfaces in the back of the neck, like you say. It contains the IFF transponder, is installed surgically, and hooks directly up to the user's nervous system. So it's not in the equipment.

Presumably, the Flood's nervous system either is not compatible with the neural interface, or else the interface rejects unknown neural systems as a fail-safe.