Abstract
This chapter discusses interactions of viruses with the animals that they infect. However, virus-induced changes at the sub-cellular and molecular levels are best studied in cultured cells; observations at this level can then be used to interpret changes found in whole animals. Viral cytopathology is as complex as cell biology itself; hence, it is not surprising that the subject is still largely at the descriptive level of understanding. The analysis of viral replication has been simplified at a biochemical level by the concept of strategies of viral replication; there is as yet no such unifying theme as to how DNA or RNA viruses redirect cellular metabolism and kill or transform infected cells. The chapter discusses the various types of interactions that can occur between virus and cell. Viruses may be categorized as cytocidal (lytic) and noncytocidal (nonlytic). Not all infections, whether cytocidal or noncytocidal, necessarily lead to the production of new virions. Cell changes of a profound nature, leading to cell death in some cases and cell transformation in others, may also occur in nonproductive (abortive) infections. Looked at from the point of view of the cell rather than the virus, certain kinds of cells are permissive, that is, they support complete replication of a particular virus, while others are non-permissive, that is, replication is blocked at some point. Cytopathic changes can occur in both productive and nonproductive infections and in permissive and nonpermissive cells.