This chapter shows Ole Maaloe heading full-speed into a study of bacteriophages, that is, viruses that infect and replicate in bacterial cells, while Niels Jerne was beginning to identify himself as an immunologist. Max Delbruck and other geneticists used the tiny phage as a model organism to understand the molecular mechanism of heredity, and during his stay at Cal Tech in the spring of 1949, Maaloe had been seriously bitten by the “phage bug.” He returned to Copenhagen full of energy and ideas and, with Orskov's indulgence, established his one-man branch of the internationally dispersed phage group in the Standardization Department. The following spring he started a series of experiments on how changes in temperature affect the reproduction of phages in the bacterial cell.