Elucidation of the mechanisms of pathogenesis underlying the diseases caused by viruses and bacteria has fascinated scientists for many years in two ways. Firstly, these pathogenic agents represent relatively sim ple biological systems for the study of basic biological processes such as replication, gene regulation, genetic variability and host-pathogen interactions. Secondly, process in this field is valuable in a practi cal sence, since it can help in the control of these diseases. The avail ability of new genetic and immunological techniques, especially recom binant DNA methods and...