Comments on Immunity to Virus Diseases *

Generalizations concerning immunity to virus diseases have required constant modification as information has developed concerning the different agents and their activities within the host. Although the viruses are similar in requiring an intracellular habitat, the great diversity in routes of infection, the tissues attacked, and the types of injury produced-necrotizing, proliferative, exudativesuggest that the mechanisms common to the immune processes may not be equally effective under all these conditions. It is not surprising to find the earlier notion that most virus infections gave rise to permanent immunity greatly revised as the natural history of the various diseases has taken form. The hypothesis was based upon inadequate information of virus diseases and was composed to a large extent of clinical and epidemiological observations that second attacks of smallpox, measles, chicken-pox, and yellow fever were rarely, if ever, recorded. In contrast,'the same type of observation noted that influenza, the common cold, and herpes febrilis gave rise to repeated illnesses. In attempting to understand why immunity is long-lived or transient following different virus infections, considerable attention has been given to the immunologic significance of the protective or neutralizing antibodies of the blood. It has become evident that their import to immunity is not the same under all circumstances and that their presence is no guarantee of complete immunity. Although the relationships of antibody and immunity to virus infection may seem confused when all infections are grouped together, when considered in terms of the mechanisms of the different infections an orderly concept tends to evolve. Moreover, explanations for dissimilarities in the duration of resistance can be discerned.