Magical Plague

"I fear that something like this could never come from nature."

- Gaius to Merlin

The Magical Plague was a disease that struck Camelot after the High Priestess Nimueh used an Afanc to poison the city's water supply.

Victims of this plague are known to develop a pale complexion, blue veins, and clouded eyes.

History
When the people of Camelot fell victim to a mysterious plague, Gaius and Merlin were called upon to investigate. They determined that the pathogen was magical in order and likely spread through water, leading Uther to order a city-wide search for evidence of sorcery. He also imposed a curfew and cordoned off the Lower Town, as that where most of the victims were from.

Though Gaius forbid Merlin from using his magic to cure the sick, when Gwen's father became infected, Merlin snuck into their house and used a magic poultice to heal him. Tom's miraculous recovery soon caught Arthur's attention and he searched the house, finding the poultice beneath Tom's pillow. Because Gwen was the only person with Tom at the time of his recovery, she was arrested for sorcery, accusing of starting the plague, and sentenced to death.

Though Merlin tried to confess what he'd done, Arthur convinced Uther that Merlin was blinded by love for Gwen and he was laughed out of the room. Fortunately, Gaius was certain they could prove her innocence by finding the true cause of the disease. Having already determined that it was spreading through water, they collected a sample from the city well and discovered that an Afanc was poisoning Camelot's water supply.

After consulting the Great Dragon, Merlin later returned to the water cavern with Arthur and Morgana and used the elements of wind and fire to destroy the Afanc. With the creature dead, the plague ended and those still sick began to recover. Gwen was later exonerated after Gaius found a shard of the Afanc's egg and recognized a symbol on it as the mark of the sorceress Nimueh, an enemy of Camelot (The Mark of Nimueh).

Trivia

 * The only known cure for the plague is the poultice that Merlin used to save Tom (The Mark of Nimueh).
 * Though most victims of the plague were peasants, a few courtiers contracted it as well (The Mark of Nimueh).