Pleomorphic microbes are microbes which have the ability to alter their shape and size, in response to environmental conditions. In 1890, Dr. William Russell found “pleomorphic” microbes, and the immune cells fighting the microbes, “wandering” inside and outside cancer tumors. The microbes had the ability to infect and change the shape of the immune cells. He was unable to culture the microbes, and referred to “the parasite of cancer”, which others later referred to as the “Russell body”. Dr. Russell was criticized for proposing an infectious cause of cancer.

Some cancers are now known to have infectious causes; however, research into infectious causes lags far behind other forms of cancer research. The research into infectious causes is primarily directed at viruses, and tends to ignore immortal bacteria and many other infectious pathogens.