From A Wiki of Ice and Fire
Greywater fever is a disease. It is possible it is endemic to the Neck, as it shares the name of Greywater Watch.
A Clash of Kings
Jojen Reed tells Bran Stark he nearly died of greywater fever during his childhood. While he was near death, he was visited in dreams by a three-eyed crow that gave him the gift of greendreams.
- ↑ 1.0 1.1 A Clash of Kings, Chapter 28, Bran IV.