Substance that is produced by bacteria and it is fairly stable. It causes fever in mammals. Pyrogen Pyrogen is broken up pieces of disease-causing substances that transition through the immune system and blood stream, while becoming problematic to the hypothalamus. Human bodily tissues and certain pathogens are also responsible for producing pyrogens. As an example, pyrogen is produced when the ultraviolet hits the DNA of a microorganisms cells, it scatters it to different pieces.