The Black Spit is a plague that struck Welltower around the time that Kaehl and Selda left. It is named after the black foam that comes from the mouths of those who suffer from it. It is highly contagious and deadly. Other symptoms include yellow, crusted eyes, a severe cough, insanity (?), and breathing trouble.
Rec and many other people from Kaehl's home floor caught the plague, including Kaehl himself after he returned to level 321 in search of his father. Kaehl eventually died to the plague.