Polyomaviruses are double-stranded DNA viruses of the family Papovaviridae. They replicate in cell nuclei, where they form 40-50-nm, non-enveloped, icosahedral nucleocapsids that exit the host cell by lysis (Fig. 1A-C). Polyomaviruses with pathogenic potential in both human and non-human hosts have been known for many decades. in recent years, however, new pathogenic polyomaviruses have been described in man, non-human primates, and other animals. New forms of acquired and iatrogenic immunocompromise have also led to changes in the prevalence and pattern of disease caused by well-characterized polyomaviruses. This presentation provides an overview of emerging polyomavirus diseases, illustrated with vignettes from our diagnostic practices. in many cases, the detection and monitoring of polyomavirus infections has been facilitated by ultrastructural examination.

Two well-characterized polyomaviruses cause disease in humans, almost invariably in the setting of immunosuppression. JC virus, described in 1971, is the causative agent for progressive multifocal leukoencephalopathy (PML), a generally fatal disorder characterized by patchy, progressive destruction of the cerebral white matter.