The human body is a complex and sophisticated machine in which all components must maintain an intricate balance to ensure optimal functioning. This balance at a specific set point is known as homeostasis. There are many homeostatic variables and the one I find most interesting is temperature, more specifically how our body responds to environmental and internal threats through thermoregulation. Many different syndromes such as heat stroke, neuroleptic malignant syndrome, malignant hyperthermia, and fever can lead to an increase in body temperature. Fever is usually triggered by an infection or inflammation while other syndromes are the result of an imbalance between heat production and loss rather than a change in the set point of body temperature (Prewitt, 2005). This article aims to describe the different elements that cause fever in the human body. As mentioned above, fever is a defense mechanism used by the immune system to eliminate invading pathogens. Infectious organisms or their products that cause fever are called pyrogens and can be exogenous (from outside) and endogenous (produced internally). They are low molecular weight proteins that modulate immune, inflammatory, and hematopoietic processes in the body (Biddle, 2006). Pyrogenicity is a fundamental biological property of several cytokines and thus the fever property links host perturbations during disease with fundamental perturbations in cell biology (Dinarello, 1999). When a microbial infection occurs, it usually causes localized tissue death or injury by stimulating the release of inflammatory mediators that attract white blood cells that engulf the pathogen and initiate the release of cytokines and prostaglandins: small proteins that facilitate the infection process. article ......3-d0c4-4249-acf9-e89dcab7dbc7%40sessionmgr10&vid=2&hid=8Dinarello, Charles A. Cytokines as endogenous pyrogens. The Journal of Infectious Diseases. 179, 2, S294-304. Retrieved from http://web.ebscohost.com.ezproxy.ccsf.edu/ehost/pdfviewer/pdfviewer?sid=7279ed45-37cf-4243-b68a-177cb17d8075%40sessionmgr10&vid=2&hid=10Dinarello, Charles A. Infection, Fever, and Exogens and endogenous pyrogens: some concepts have changed. Journal of Endotoxin Research. 10, 4, 201-218. Retrieved from http://web.ebscohost.com.ezproxy.ccsf.edu/ehost/pdfviewer/pdfviewer?sid=4557325b-67c2-4cf7-8180-e64f25901cb6%40sessionmgr14&vid=2&hid=10Prewitt, Ellen M. Fever: Fact, Fiction , Physiology. Critical Care Nurse, February 2005, 8-16. Retrieved from http://web.ebscohost.com.ezproxy.ccsf.edu/ehost/pdfviewer/pdfviewer?sid=99f55fc0-18eb-4a99-89c6-58dbd9359ef9%40sessionmgr4&vid=2&hid=8