From Lost Fables Wiki
A chemical imbalance within the blood of the Ie
Only Ie who stray from their preferred icy biomes may be affected, though they all carry the disease, just waiting for it to pounce.
Heartthaw is, ninety-nine percent of the time, merely an annoyance. After spending a single phase (1 IRL Day) away from low temperatures, the inflicted begins to suffer a more or less perpetual low-grade headache, and much greater irritability. Only temperatures above 4 degrees celsius will cause this condition. If one persists for another six phases in these temperatures, they will begin to occasionally suffer heatstroke.
This illness will not kill you directly. The le will suffer withdrawal symptoms and heatstroke while they remain outside icy biomes. Confusion, agitation, slurred speech, irritability, delirium and seizures can be caused by heatstroke.
Le suffering under this imbalance can be treated before the age is over, after which it's irreversible. The Heartthaw is prevented by smoking or the consumption of wintermint, an herb which tastes like extreme cold, sharp and crisp mint to balance out the body. As a side effect, this plant has many of the same effects on an le that tobacco use does on a human, though with no recorded downsides, besides addiction, and so smoking it is extremely prevalent in their culture to avoid any possibility of Heartthaw. The easiest preventative measure would be to stay in icy biomes to prevent the disease in any capacity, having to remain in the Tundra for one fourth (1/4th) the time out to recuperate their bodies.