 In the same year, Heinz Fraenkel-Conrat and Robley Williams showed that purified tobacco mosaic virus RNA and its coat protein can assemble by themselves to form functional viruses, suggesting that this simple mechanism was probably the means through which viruses were created within their host cells.By 1935, many plant diseases were thought to be caused by viruses.In 1922, John Kunkel Small (1869–1938) discovered that insects could act as vectors and transmit virus to plants.