As noted earlier, in Chapter 4, biological factors as causes of cancer in lower animals have been known since the beginning of the twentieth century and were suspected even earlier. However, it was not until the latter half of the twentieth century that infectious agents began to be signifi- cantly appreciated as causative factors in human cancer. Perhaps the scientific embarrassment engendered by the irreproducibility of Febiger’s experiment (Chapter 4) led to an aversion of scientists to try and relate infectious agents to the development of the neoplastic process. This was true despite the suggestion from ancient times of an association between infection with spe- cies of platyhelminth worms, especially Schistosoma haematobium, and bladder cancer; this had been known or suspected for thousands of years beginning in ancient Egypt (cf. Elsebai, 1977; Hicks, 1983). That bacteria and viruses could cause human cancer was not substantially appreci- ated until the last four decades of the twentieth century.