In the books
The Great Spring Sickness was a plague epidemic that followed the Great Spring of 209AL, killing tens of thousands in the Seven Kingdoms. It killed tens of thousands, specially in the major cities; it was bad in Lannisport, worse in Oldtown, but worst of all in King's Landing, where four in ten succumbed to it. A strong man could wake up healthy in the morning and die by the evening. Dorne and the Vale of Arryn did not suffer from the epidemic, as they closed off their access roads and their ports to all travelers.
King Daeron the Good, his last Hand of the King, and his two most immediate heirs, Princes Valarr and Matarys, as well as the High Septon, a third of the Most Devout, and nearly all of the Silent Sisters in King's Landing. Lord Brynden Rivers, the Hand of the King to Daeron's successor, Aerys I Targaryen, ordered the many bodies be brought to the Dragonpit and burned by pyromancers. The light of their wildfire pyres could be seen as a glow throughout the city during the night.