Chills are a sensation of coldness accompanied by shivering, often occurring when the body’s temperature regulation system responds to infection, fever, or environmental exposure. They result from rapid muscle contractions that generate heat as the body attempts to raise its core temperature to fight illness. Chills commonly accompany infections such as influenza, pneumonia, urinary tract infections, or other systemic illnesses, and may be associated with fever, sweating, fatigue, or body aches. In some cases, chills can also occur without fever due to anxiety, hypoglycemia, or exposure to cold environments. Persistent, severe, or unexplained chills—especially when accompanied by high fever or other concerning symptoms—should prompt medical evaluation to determine the underlying cause.