HILARY KOPROWSKI 
wish to enter into the controversy over the role of tobacco 
smoking in carcinogenesis, but it is clear that numerous cancer- 
inducing environmental factors exist today and many will be 
discovered in the future which may be of equal or perhaps 
greater importance. Although removal of such factors from the 
environment will be one of the prophylactic measures employed, 
it must be recognized that it is in our power to eliminate only 
those factors which have caused cancer during the past few 
decades and not those which will cause it in the coming twenty 
years. Thus the success of these prophylactic measures will be 
significantly limited, and the generations to come will have to 
derive satisfaction from the knowledge that they would have 
been able to prevent some form of cancer in their parents, if 
they were still living, even though they may not be able to 
avoid exposure to environmental factors which might cause 
malignancy in themselves or their children. 
Up to this year there was little chance that any procedure 
based on vaccination of man against a virus infection might 
bring about immunity against cancer, even though it had been 
known for some time that immunization of rodents against a 
tumour virus was successful in preventing growth of tumours 
originally induced by these viruses. However, this year evidence 
has been presented that three viruses, two involved in certain 
types of respiratory infections of man and one present in 
kidneys of normal monkeys, caused tumour formation in 
hamsters and one of these viruses transformed human cells in 
tissue culture. Thus it is not impossible that in the future viral 
agents known to cause disease of trivial nature in man may prove 
to be carcinogenic. 
Although a direct link between these events and tumour 
induction in man may not be established for some time to come, 
the possible identification of a specific antigen in human cancer 
cells related in some fashion to the antigenic components of the 
virus or its essential constituents may raise the possibility of 
immunization of man against the virus and prevention of a 
certain type (probably only one type) of malignancy. An 
epidemiological study in which incidence of malignancy will be 
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