The papillomas caused by the Shope virus sometimes grow down spontaneously into the subcutaneous tissue and extend along the lymphatics in the same way as do many cancers of the human breast. They may even invade the voluntary muscle under such circumstances, taking on an aspect suggestive of squamous cell carcinomatosis, but ultimately they differentiate in the way characteristic of the papilloma. Slight operative interferences with papillomas may be followed by a development of secondary nodules in the lungs. These result from cell emboli, and the same local conditions determine their fate as are effective in the case of emboli composed of human cancer cells. The virus-induced papilloma is not only a neoplasm in its immediate aspect and habit but sometimes one that verges upon malignancy. The tumors, including the cancers, which eventually derive from it in favorable hosts, are representative of more than a mere enhancement of the activity of the growth. They develop within a relatively brief period of time but only after the papilloma has grown for a long while; and they are morphologically various whereas the parent tumor is remarkably constant in its form. Some of the new growths differ but little from the papilloma, however, even when possessed of the ability to metastasize, and many continue to be influenced by the virus.
The Shope virus is heavily conditioned in its carcinogenic activity, yet it is the nearest cause for cancer now known.