A sheep or livestock disease. Lacking a sophisticated system of classifying animal diseases, contemporaries lumped all outbreaks as murrains. (Waugh, Scott. England in the Reign of Edward III, 238)
Murrain is a highly infectious disease of cattle and sheep. It literally means "disease" and was used in medieval times to represent just that. The population of that era had no way of identifying specific diseases in their livestock so simply put all illnesses under one heading.