The wor pyrogen, which can be traced to the Greek pyro, meaning burning or fire and gennao, meaning to make or to create, is now used as an apt description for substance that produce elevated body temperature. Pyrogens are usually bacterial products and remains or decaying products of the bacterial cell walls. Even in minimum dose, these substances induce elevated body temperature when injected into humans and animals. Pyrogens are usually high-molecular-weight substances of polymerous nature, like lipopolysacharids. Pyrogens could be either microbial or non-microbial.