This brief survey of bacteriophage infection has provided support for an obvious but important generalization: that the study of virus infection at the cellular level is a branch of cellular genetics. The interactions between viral and cellular functions show that infection with a virus is not just a disrupting intrusion; it is an addition to the cellular endowment of genetic specificity. The outcome of this addition depends on the nature of the instructions carried by the viral genome, on the ability of the cellular machinery to carry out these instructions, and on the availability of control mechanisms to regulate the functions of the added element.