Rigor is a shaking

Tremor

A tremor is an involuntary, somewhat rhythmic, muscle contraction and relaxation involving to-and-fro movements of one or more body parts. It is the most common of all involuntary movements and can affect the hands, arms, eyes, face, head, vocal folds, trunk, and legs. Most tremors occur in the...

 occurring during a high fever

Fever

Fever is a common medical sign characterized by an elevation of temperature above the normal range of due to an increase in the body temperature regulatory set-point. This increase in set-point triggers increased muscle tone and shivering.As a person's temperature increases, there is, in...

. It occurs because cytokine

Cytokine

Cytokines are small cell-signaling protein molecules that are secreted by the glial cells of the nervous system and by numerous cells of the immune system and are a category of signaling molecules used extensively in intercellular communication...

s and prostaglandin

Prostaglandin

A prostaglandin is any member of a group of lipid compounds that are derived enzymatically from fatty acids and have important functions in the animal body. Every prostaglandin contains 20 carbon atoms, including a 5-carbon ring....

s are released as part of an immune response and increase the set point for body temperature in the hypothalamus

Hypothalamus

The Hypothalamus is a portion of the brain that contains a number of small nuclei with a variety of functions...

.

It is sometimes considered a synonym for "chills".

Physiology

The increased set point causes the body temperature to rise (pyrexia), but also makes the patient feel cold until the new set point is reached. Rigor occurs because the patient's body is effectively shivering in a physiological attempt to increase body temperature to the new set point.

Causes

Rigors are commonly caused by:

• Influenza
  Influenza
  Influenza, commonly referred to as the flu, is an infectious disease caused by RNA viruses of the family Orthomyxoviridae , that affects birds and mammals...
  
  
• Tonsillitis
  Tonsillitis
  Tonsillitis is an inflammation of the tonsils most commonly caused by viral or bacterial infection. Symptoms of tonsillitis include sore throat and fever. While no treatment has been found to shorten the duration of viral tonsillitis, bacterial causes are treatable with antibiotics...
  
  
• Infectious Pneumonia
  Pneumonia
  Pneumonia is an inflammatory condition of the lung—especially affecting the microscopic air sacs —associated with fever, chest symptoms, and a lack of air space on a chest X-ray. Pneumonia is typically caused by an infection but there are a number of other causes...
  
  
• Biliary sepsis (part of Charcot's triad 2
  Charcot's triad 2
  Charcot's cholangitis triad is the combination of jaundice; fever, usually with rigors; and right upper quadrant abdominal pain. It occurs as a result of ascending cholangitis . When the presentation also includes hypotension and mental status changes, it is known as Reynolds' pentad...
  
  )
• Pyelonephritis
  Pyelonephritis
  Pyelonephritis is an ascending urinary tract infection that has reached the pyelum or pelvis of the kidney. It is a form of nephritis that is also referred to as pyelitis...
  
  
• Visceral abscess
  Abscess
  An abscess is a collection of pus that has accumulated in a cavity formed by the tissue in which the pus resides due to an infectious process or other foreign materials...
  
   (including lung, liver and paracolic)
• Malaria
  Malaria
  Malaria is a mosquito-borne infectious disease of humans and other animals caused by eukaryotic protists of the genus Plasmodium. The disease results from the multiplication of Plasmodium parasites within red blood cells, causing symptoms that typically include fever and headache, in severe cases...
  
  
• A side effect of treatment with amphotericin B
  Amphotericin B
  Amphotericin B is a polyene antifungal drug, often used intravenously for systemic fungal infections...
  
  
• General Illnesses