So you are saying in effect that cancer is contageous - or at least the viruses that cause cancer are contageous. In that case why was there not plagues or at least large outbreaks of cancer in areas where the population was crowded together? Both nutrition and hygeine were sorely lacking until after the turn of the 20th century in much of the world, yet cancer has developed in the modern industrial world in parallel with chemical pollution.
No doubt that lack of nutrition and immuno suppressing drugs have played a role over the past half century, but cancer was already on the rise before either of those came into play, as was industrial pollutants, chemicals, pesticides and herbicides.
I think that manmade chemicals and heavy metal pollution are clearly prime causes of cancer, perhaps in combination with viruses, but a necessary causal factor in the rise of cancer.