The first victim of the plague.
A plague struck Camelot when the sorceress Nimueh attacked the city's water supply using an Afanc.
The egg of the Afanc was delivered to Camelot's watersource by way of a large stone fount in a cave that connected with Camelot's water source by way of a complex underground cave system. The plague killed several dozen residents of Camelot. Most of them were peasants, while a few were courtiers. Gaius and Merlin investigated the disease, and Gaius determined that the pathogen was magically manufactured. When informed of the magical nature of the disease, Uther ordered the entire city searched for evidence of the sorcerer, and cordoned off the Lower Quarter.
When Gwen's father became infected with the disease, Merlin defied Gaius's instructions and used magic to create a poultice to cure him. Gwen's father recovered, but his recuperation raised the suspicion's of Prince Arthur, who discovered that Gwen was the only person with her father during his miraculous recovery. Arthur searched Gwen's house and discovered the poultice, and Gwen was sentenced to death by burning for practicing witchcraft.
Merlin attempted to take responsibility for the poultice, but Arthur convinced his father that Merlin was blinded by his love for Gwen. After discerning the origin of the disease, Merlin and Gaius descended to Camelot's water source and encountered the Afanc; Merlin then sought the counsel of The Great Dragon, who told him that he and Arthur were two sides of a coin, and advised him to "use the Elements" at his disposal. Gaius explained the nature of the Elements to Merlin, who then returned to the water source with Arthur and Morgana and destroyed the Afanc.
The corpses because of the plague
With the plague ended and the Afanc destroyed, Gaius found the shards of the Afanc's egg, which bore the mark of Nimueh. Gwen was thus exonerated and released from prison (The Mark of Nimueh).