The bloody flux, also known as the "Pale Mare" in Meereen, is a disease that spreads easily. Its symptoms are fever, intestinal hemorrhages, and diarrhea.
See also Medicine.
After the Riot of King's Landing, the bloody flux was spreading through the pot shops of Pisswater Bend in King's Landing.
The Bloody flux spreads through Meereen and the Yunkai army besieging it. The disease was brought over by a dying Astapori rider atop a pale mare, which gave the plague its nickname.