VIRUSES, CANCER, GENES, AND LIFE — STANLEY 365 



are times when a specific genetic element of a cell can be freed of the 

 normal controlling mechanism of the cell and go forth in viable form 

 in solution or associated with a virus, enter a different cell, replace 

 a homologous chromosomal segment, and resume its original specific 

 function in the new cell. It is obvious that the latter phenomenon 

 could readily be considered an infectious process, and that viruses 

 can act as genes and genes as viruses under certain circumstances. 



I should now like to discuss the relationships which involve cancer. 

 You probably know that cancer or abnormal, uncontrolled cellular 

 growth may occur in all kinds of organisms and that cancer is second 

 only to heart disease as a killer of mankind ; hence I need say no more 

 about the relationship between cancer and life. Cancer originates 

 when a normal cell for reasons, some known and some unknown, sud- 

 denly becomes a cancer cell which then multiplies widely and with- 

 out apparent restraint. Cancer may originate in many different kinds 

 of cells, but the cancer cell usually continues to carry certain traits of 

 the cell of origin. The transformation of a normal cell into a cancer 

 cell may have more than one kind of a cause, but there is good reason 

 to consider the relationships that exist between viruses and cancer. 

 Viruses have been implicated in animal cancers ever since Peyton 

 Kous, in 1911, transmitted a chicken sarcoma from animal to animal 

 by means of a cell-free filtrate. Despite the fact that today viruses 

 are known to cause cancer or tumors in chickens, pheasants, ducks, 

 mice, frogs, rabbits, deer, and other animals, and even in certain 

 plants, there exists a great reluctance to accept viruses as being of 

 etiological importance in human cancer. However, basic biological 

 phenomena generally do not differ strikingly as one goes from one 

 species to another, and I must say that I regard the fact, now proved 

 beyond contention, that viruses can cause cancer in animals to be 

 directly pertinent to the human cancer problem. It should be recog- 

 nized that cancer is a biological problem and not a problem that is 

 unique for man. 



Since there is no evidence that human cancer as generally experi- 

 enced is infectious, many persons believe that because viruses are in- 

 fectious agents they cannot possibly be of etiological importance in 

 human cancer. However, this is not a valid conclusion for several 

 reasons. It is well known from the work of Bryan and of Beard that 

 animal cancer viruses may alternately be filterable and hence infec- 

 tious and then nonfilterable and hence appear noninfectious, appar- 

 ently owing to great variations in the actual amount of virus present 

 in the cancer. It is also well known that viruses may be highly spe- 

 cific, so specific in fact that a given virus may infect and cause disease 

 only in one kind of cell in one kind of animal and hence, under all 

 other conditions, appear noninfectious. For example, the kidney 



