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How did Peyton Rous show that a virus could cause cancer in chickens?

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Diagram illustrating the process of retrovirus infection and reverse transcription, showing a retrovirus entering a host cell, releasing viral RNA, reverse transcriptase converting RNA to DNA, integration into the host genome, transcription of new viral RNA, assembly of new virions, and budding from the cell. — Reverse transcriptase After a retrovirus infects a cell, an enzyme known as reverse transcriptase converts viral RNA into proviral DNA in the cytoplasm. Proviral DNA then enters the nucleus, where it is incorporated into the host cell's DNA.

In 1911, American pathologist Peyton Rous demonstrated that cancer could be transmitted by a virus through experiments with chickens. He observed that healthy hens developed malignant sarcomas—cancers of the connective tissue—after being injected with tumor cells from other chickens.12

Rous further investigated these tumor cells and successfully isolated a submicroscopic agent, later identified as the Rous sarcoma virus, which was capable of transmitting the disease to other fowl of the same inbred stock. This discovery provided early evidence for the viral theory of cancer causation, though the scientific community was initially skeptical of his findings. It was not until decades later that the significance of his work was fully recognized, leading to his receipt of the Nobel Prize in 1966.12