 Dear students, in this topic we shall discuss fever. Dear students, the hypothalamic thermoregulatory center is sensitive to certain chemicals which cause fever. Such substances are collectively termed as pyrogens. So pyrogens are the fever causing substances. Based on their origin, the pyrogens belong to two different classes or categories. They may be exogenous pyrogens or endogenous pyrogens. The exogenous pyrogens are produced by gram-negative bacteria. These chemicals are heat stable, high molecular weight polysaccharides. They are very potent. If only a billionth part of a gram of purified exogenous pyrogens toxin is injected into a large mammal, it will cause an elevation of its body temperature. Dear students, the endogenous pyrogens arise from animals own tissues. These are the compounds which are actually heat labile that is heat sensitive proteins. They are released by leucocytes in response to the circulating exogenous pyrogens. When exogenous pyrogens are produced by some bacteria, then the immune system of the body, which is important for leucocytes, produces pyrogens which are called endogenous pyrogens in comparison to the exogenous pyrogens. These endogenous pyrogens act directly on the hypothalamic thermostatic center to raise the body temperature. Exogenous pyrogens directly thermostatic center, but they act indirectly by stimulating the release of endogenous pyrogens to raise the body temperature. Dear students, the pyrogen sensitivity of hypothalamic temperature sensing neurons causes elevation of the set temperature to a higher than normal level. For example, our thermostatic center has set the temperature of our body at 37 degree centigrade or 98.6 degree Fahrenheit. However, when any pyrogen is produced in our body, it raises this T-set or hypothalamic T-set. For example, it sets it on 100 degree Fahrenheit or 38 degree centigrade. So, our body temperature elevates to that level and this elevation of body temperature is known as fever. Dear students, fever means the rise of body temperature that has certain adaptive significance because the elevated body temperature that has bacterial static effect, the temperature rises because the growth in the body decreases at a higher temperature. Therefore, the body increases its temperature.