This is when you and your parner fuck when one or both of you is ill.
You and your partner are dirty nimphos and don't care you get fucking coughed over, sneezed on... or have nose dribble drip on you.
You can ignour or even enjoy the desgusting sound of your parner chocking on their own phlem.
Guy1: Me and the wife are ill, but we still had great sex last night...
Guy2: Ewww! You dirty bastards had Flusex!