Why the Body Shakes During Infection
Shaking (shivering) during infection is a thermoregulatory response triggered when the preoptic area (POA) of the hypothalamus receives inflammatory signals from the infection, causing it to reset the body's temperature set-point upward to create fever, and shivering is the mechanism to rapidly generate heat to reach this new elevated temperature target.
Physiologic Mechanism
The body's shaking response during infection involves a coordinated neural pathway:
The preoptic area (POA) of the hypothalamus serves as the central thermoregulatory control center, monitoring body core temperature and receiving peripheral humoral signals from systemic infection 1.
When infection occurs, inflammatory mediators (pyrogens) act on neurons in the POA, transmitting afferent information about the systemic infection and triggering the POA to provide descending command signals to thermoregulatory effectors 1.
Shivering thermogenesis is one of three primary involuntary thermoregulatory responses initiated by the POA, along with nonshivering brown adipose tissue thermogenesis and cutaneous vasoconstriction 1.
The Fever Response
The shaking specifically occurs during the "chill phase" of fever development:
The POA resets the body's temperature set-point upward in response to infection, creating a mismatch between current body temperature and the new target temperature 1.
Shivering is the body's rapid heat-generation mechanism to quickly elevate core temperature to match the newly elevated set-point established by the hypothalamus 1.
This explains why patients feel cold and shake despite having an elevated temperature—their brain has temporarily redefined "normal" temperature to a higher level.
Clinical Context in Sepsis
In severe infections like sepsis, temperature dysregulation is a cardinal manifestation:
Temperature >38°C or <36°C is one of the four SIRS (systemic inflammatory response) criteria used to define sepsis 2.
The shaking/shivering represents the body's attempt to generate the fever, which is part of the immune response to infection 2.
Special Consideration: Post-Antibiotic Reactions
A specific type of shaking can occur after antibiotic treatment of certain infections:
The Jarisch-Herxheimer reaction causes shaking chills within 24 hours after antibiotic treatment of spirochetal infections (syphilis, Lyme disease, leptospirosis, relapsing fever) 3.
This reaction is mediated by pro-inflammatory cytokines (TNF, IL-6, IL-8) released when antibiotics cause rapid spirochete death and is distinct from the initial infection-related shivering 3.