Why does infection cause fever? by Connie b. Dellobuono
Answer by Connie b. Dellobuono:
When people are ill (infection or inflammation related), their body's immune system fights the disease, and so the body temperature rises. Fever is a defensive measure of the body against the germs: the life cycles of the germs are disrupted when the body temperature rises.
A fever is when a person's body temperature is hotter than 37.5 degrees Celsius (99.5 Fahrenheit). Normal body temperature for humans varies based on a variety of factors, including age and level of physical activity.[1] It is typically cited as 37 degrees Celsius (98.6 F), but naturally varies from person to person by at least .5 degrees Celsius. The actual measurement of body temperature will vary based on the location of the measurement. For a temperature taken from under the tongue, the measurement may be lower. Rectal temperatures will read about 0.3 C (0.55 F) higher, and armpit temperatures will read about the same amount lower.
Source: Wiki
Fever can be caused by factors outside or inside the body. Microorganisms, including bacteria and parasites, can produce chemical poisons. Both the microorganism and the poisons cause the white blood cells (called monocytes) to produce substances called pyrogens. It's the pyrogens that actually cause the fever.
The body also produces pyrogens in response to infection, inflammation, cancer, or an allergy. Illnesses in which the body's immune system attacks its own tissues (called an autoimmune disease), such as rheumatoid arthritis, can also cause fever. Too much exercise in hot weather, overexposure to sunlight, or some medications can cause a fever that is a medical emergency. In these situations, get immediate medical attention.
Symptoms and Complications
When the body is fighting an injury or infection, the hypothalamus (a part of the brain) sets the body temperature at a higher level. The body compensates for this by moving blood away from the skin so the amount of heat lost through the skin is reduced.
The muscles might repeatedly contract to keep the body warm, which causes shivering. When the blood that is warmed up to the new temperature reaches the hypothalamus, these symptoms usually stop, and just the fever remains. When the body's thermostat is set back to its normal temperature, it moves the blood back to the skin and excess heat is lost through sweating. Sometimes chills occur when this happens.
Source: http://bodyandhealth.canada.com/condition/getcondition/Fever