The Mildew (or the Rot) is a disease that exists in Chromatacia and is always swiftly fatal once contracted. Eddie Russett's mother died of the Mildew.
There are many different types of Mildew, but none can be cured, and the sufferer is sent to the Green Room, where they pass out from an excess of Green. However, this treatment doesn't work for Greens.
Some time ago there had been a mildew outbreak in Rusty Hill. The town had quickly been abandoned.
Mildew is caused by seeing a certain colour, a sort of greeny-red that Jane calls "Greed".