"The 1940s and 1950s were marked by intense debates over the origin of drug resistance in microbes. ... Antibiotic resistance became a key issue among those disputing physiological (usually termed ‘’) vs. genetic ( and ) explanations of variation in . Postwar developments connected with the gave this debate a new political valence. Proponents of the weighed in with support for the genetic theory. However, certain features of drug resistance seemed inexplicable by mutation and selection, particularly the phenomenon of ‘multiple resistance’—the emergence of resistance in a single strain against several unrelated antibiotics. In the late 1950s, and his collaborators solved this puzzle by determining that resistance could be conferred by rather than . These could carry resistance to many antibiotics and seemed able to promote their own dissemination in bacterial populations. In the end, the vindication of the genetic view of drug resistance was accompanied by a recasting of the ‘gene’ to include extrachromosomal hereditary units carried on viruses and s."