User blog comment:Mckrongs/Monster Hunter 4 for the Nintendo 3DS/@comment-4813670-20130916084755/@comment-11685710-20130916103641

When the purple gauge is full, the Hunter gets "infected". This means, that he won't recover red health and it boosts the attack quite a bit (and I think the affinity, too). I'm not sure if you recover from it like the soiled status, but you can heal it/prevent the gauge from filling with nullberries.