Rigor (medicine)
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A rigor is an episode of shaking occurring during a high fever. It occurs because cytokines and prostaglandins released as part of an immune response increase the set point for body temperature in the hypothalamus.
The increased set point causes the body temperature to rise (pyrexia), but also makes the patient feel cold until the new set point has been reached. Rigors occur because the patient is effectively shivering in a physiological attempt to increase body temperature to the new set point.