to maintain and transplant the tumor in different individuals. In 1911, he made a seminal observation that cell-free filtrate (using Berkefeld filter that separate bacteria and large microbes) of chicken sarcoma could produce a malignant tumor when transferred to other chickens, describing:A transmissible sarcoma of the chicken has been under observation in this laboratory for the past fourteen months, and it has assumed of late a special interest because of its extreme malignancy and a tendency to wide-spread metastasis... small quantities of a cell-free filtrate have sufficed to transmit the growth to susceptible fowls. This finding, that cancer could be transmitted by a virus (now known as the Rous sarcoma virus, a retrovirus), was widely discredited by most of the field's experts at that time as "utter nonsense" as it was a medically accepted fact that cancer was not an infection. As recorded by Charles Oberling:Tumor pathology was then completely under the spell of the German school of pathologic anatomy which, probably as an aftermath of the antagonism between Robert Koch and [Rudolf] Virchow, was utterly opposed to any theory of an infectious origin of cancer. And suddenly, in opposition to all these dignified and bearded Herren Professoren who firmly believed what they said, rose the voice of a young American who claimed to have transmitted by a cell-free filtrate a neoplasm—a chicken sarcoma. Of course this could not be true, and for years they did not even try to repeat his experiments.He was even accused of using faulty technique and contaminating the tumor samples with cancer cells. However, he was convinced that the malignancy was as those of any other cancer cells, the only difference being that it could be produced by a cell-free filtrate of a tumor. Experiments he continued with James B. Murphy and published made conclusive evidences for the cancerous nature of the infection. An experiment they did with W.H. Tytler in 1912 gave the first clue of virus as the filterable agent, but failed to make an exact identification. Rous continued to work on cancer up to 1915, after which he gave up due to failure to obtain other carcinogenic agents from chicken and mice, and general acceptance of his discovery. After 18 years, he returned to cancer research upon the request of a colleague Richard Shope. In 1933, Shope had discovered that a skin tumor (papilloma) in cottontail rabbits was due to a filterable