T cell responses to viral infections can mediate either protective immunity or damaging immunopathology. Viral infections induce the proliferation of T cells specific for viral antigens and cause a loss in the number of T cells with other specificities. In immunologically naive hosts, viruses will induce T cell responses that, dependent on the MHC, recognize a distinct hierarchy of virus-encoded T cell epitopes. This hierarchy can change if the host has previously encountered another pathogen that elicited a memory pool ofT cells specific to a cross-reactive epitope. This heterologous immunity can deviate the normal immune response and result in either beneficial or harmful effects on the host. Each host has a unique T cell repertoire caused by the random DNA rearrangement that created it, so the specific T cells that create the epitope hierarchy differ between individuals. This "private specificity" seems of little significance in the T cell response of a naive host to infection, but it is of profound importance under conditions of heterologous immunity, where a small subset of a cross-reactive memory pool may expand and dominate a response. Examples are given of how the private specificities of immune responses under conditions of heterologous immunity influence the pathogenesis of murine and human viral infections.