Publisher Summary A virus may have oscillated in the course of evolution, between being an independent parasite and a cell constituent. It is wrong to think of the evolution of methods of transmission as proceeding always in one direction. Bacteria infecting insects by the horizontal route could have evolved into symbionts, transmitted vertically, and becoming dependent on the interior of the cell for their nourishment. In attempting to piece together the story of virus evolution, it is the varying methods of successful attack and defense which particularly engage the attention. In the field of virus transmission one can obtain the clues about virus evolution: this aspect of the subject is discussed in this chapter. The chapter also explores the immunological aspect of the virus evolution. The viruses can multiply and vary and therefore evolve much more rapidly than their victims. Viruses are able to circumvent defensive weapon of victims. They may, as with mosquito borne yellow fever, get from one host to another before the antibodies of the first host have had time to develop. They may remain for long periods within cells where antibody cannot get at them, as with rabbit papilloma. They may make their attack on a mucous surface without having to enter the blood stream, as the common cold does, and thus by-pass any antibodies the body may have prepared, or they may achieve vertical transmission. It is also found that passing viruses serially in the host can enhance the virus virulence and mutants have greater powers of multiplication.