Genetics has a tradition that dates back to the Ancient Greeks. It developed, between insight and contradiction, from the post-Renaissance to the mid-1800s, when Mendel and Darwin gave it the first experimental and conceptual bases. From 1910, genetics became a true experimental discipline of Biology thanks to the work of Morgan's group. On the contrary, virology is a relatively young discipline which had origin only after the success of the "germ theory" of Pasteur and Koch, by the hypothesis of the contagium vivum fluidum of Beijerinck, in 1898. In spite of their historical difference, the modern development of the two disciplines had a close connection. In 1922, the geneticist Muller first compared the bacteriophage to the gene and, in 1923, the (phyto)physiologists Benjamin Duggar and Joanne Karrer Armstrong suggested the analogy between gene and Tobacco mosaic virus (TMV). Knowledge on the biochemical nature of gene and virus developed in the early 1940s when the biochemists began to suspect that the nucleic acids might be the genetical determinants for both the bionts. Avery and co-workers discovered in 1944 that DNA was the principle of the transmission of hereditary characters in bacteria and, in 1948, a little group of English (phyto)virologists (Markham, Matthews and Smith) discovered that the RNA of a plant virus (Turnip yellow mosaic virus) was directly involved in virus replication. The fundamental significance of the two discoveries was not gathered by geneticists and virologists, even because the respective groups did not gave the necessary emphasis to their results. Thus, the discovery of the role of the nucleic acids in virus replication is historically attributed to Hershey and Chase for DNA phage, and to Fraenkel-Conrat and the German virologists Gierer and Schramm for plant viruses.