Viral pathogenesis depends on viral and host factors. Viral infections may be lytic, where the virus replicates an destroys cells of the host. Alternatively, the infection may be latent, at which time the viral genome (the viral nucleic acid) is maintained in the infected cell, but new virus is not made. Such latent infections may be maintained for long periods of time, in part because there is no clear target for the host immune system or for antiviral medications. Human immunodeficiency virus (HIV) infection and the herpes virus infections herpes simplex virus (HSV) and varicella-zoster virus (VZV) establish well-described latent infections.

HSV and VZV latent infections are infections of neurons. With reactivation infectious virus is produced and clinical infections, cold sores and genital infection by the former, and shingles by the latter. Latent infections are not low-level infections but are qualitatively different, with very limited expression of viral functions.

Latent infection by HSV and VZV are infections of neurons of the peripheral nervous system. There is some evidence that HSV may establish latent infection of the brain. If so, a possible relationship to many types of human illnesses may need be considered.