Cell-mediated immunity, which encompasses T-cell responses, plays a principal role bringing viral infections under control. T cells accomplish this by elaborating effector functions, such as the production of chemokines and cytokines, which can have direct as well as indirect antiviral effects, and also assist in regulating the overall immune response. Certain effector T cells can kill virus-infected cells through cell-to-cell contact and in this way provide an important means of destroying the host's cells which serve as the production sites of progeny virus. Cell-mediated immune responses are not only effective during acute phase of viral infections but also establish long-lived immunological memory. These memory T cells become rapidly activated if they re-encounter virally infected cells and therefore function to protect against secondary exposures to the original viral pathogen. Although many viral infections are cleared from the host, certain viruses can persist indefinitely following infection. In these instances T-cell responses often bring about some level of initial containment of the infection and operate to control the infection at a steady-state level over time. Cell-mediated immune responses during viral infections are not always beneficial as they can cause damage to host tissues which manifests as immunopathology.