The Plague ability used to be called Disease. The Disease ability used to work quite differently but was scrapped and later replaced with the current version. This was its description: Inflicts a single enemy unit with a contagious disease causing damage over time, after a short period of time the disease spreads to any nearby units. Units that survive the disease are left with about 40% hp. As of the WWI build the disease was very difficult to spot (tiny green cloud), making it extremely powerful. It affects both mechanical and bioligical units, as well as Protoss shields.