Binder's Chills is a dangerous result that can occur when using sympathy recklessly. If a sympathist uses his own body heat to perform sympathy, especially from his blood, then the body can cool down very rapidly which can upset the entire system. When heat is lost from the blood, the body cools as a whole unit leading to potentially fatal results due to the drastic temperature loss. A mild case of chills is just chilling. But more extreme cases can lead to shock and hypothermia.
Flesh is another source that a sympathist can use in order to draw from one's own body heat. Flesh is advantageous in the sense that it can stand a more drastic temperature loss than blood can, but a sympathist might be tempted to use blood as it offers up heat more rapidly than flesh.
Because of this, sypathists always use an external source (usually fire) whenever possible, and only draw from their own body as a last resort.
Elxa Dal states to his Advanced Sympathy class that it is safe for a sympathist to draw up to one and a half degrees from his body.
When Fenton duels Kvothe in The Name of the Wind, he draws approximately eight or nine degrees from his blood, which gives him the binder's chills. Under the care of Elxa Dal and the Medica, Fenton makes a full recovery.