The living organisms that thrive in the warm, wet, Petri dish-like conditions of a hot tub (especially communal tubs). In more favorable conditions, jacooties can actually be seen floating in the froth. DO NOT assume that skimming the froth will remove jacooties from the tub.
I'm not going in that hot tub. Andrew was in there last night with two skanks and now it's full of jacooties. Look, you can see them circling in the froth.