But then Rous stumbled on an even more peculiar result. Shuttling tumors from one bird to another, he began to pass the cells through a set of filters, a series of finer and finer cellular sieves, until the cells had been eliminated from the mix and all that was left was the filtrate derived from the cells. Rous expected the tumor transmission to stop, but instead, the tumors continued propagating with a ghostly efficacy—at times even increasing in transmissibility as the cells had progressively vanished. The agent responsible for carrying the cancer, Rous concluded, was not a cell or an
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