The gas is musky as you run, unable to hold your breath as long as you needed to. You try to speed up, to perhaps flee fast enough for nothing to happen, but it's already too late. Your insides tingle, something along your back sprouting through skin turning to slime. A beak forms on your face, and you nearly trip as your legs bend wrong, shifting into something animalistic.
You feel nauseous despite it all being painless, your fingers melting into feathers and a tail of some sort breaking through your shorts. The tingling feeling gets stronger, and you feel something grow from your head, releasing more spores into the air and speeding up the last remnants of your change....