What is a rigor and why does it occur?

A rigor is characterized by violent shivering that typically occurs in conjunction with a fever. This involuntary muscle contraction is the body's response to rapidly rising body temperature, often as a result of infection or illness. When the body's set-point temperature is raised, the body reacts by generating heat through muscle activity, leading to shivering.

During a fever, the hypothalamus, which regulates body temperature, triggers this reaction to increase body temperature to help fight off pathogens. It's important in the context of infections or inflammatory conditions, where the body is mounting an immune response. Thus, understanding rigor in this way highlights its role as a physiological mechanism in response to illness, particularly in the face of infections where fevers are common.

The other options describe conditions that do not align with the medical definition of rigor or the body's physiological responses. For example, dehydration can cause a range of symptoms but is not inherently linked to shivering. Cold weather may induce shivering as a separate thermoregulation response but does not define rigor. Excessive muscle fatigue can present with tiredness or weakness but is not characterized by shivering either.