The number of polyomaviruses causing infections in humans is as high as thirteen. The BK and JC polyomaviruses and the diseases caused by them are best known. For the present, the Merkel cell polyomavirus is the only human polyomavirus considered to be a causative agent of cancer. Other disease associations of polyomaviruses are also subject to active research. All polyomavirus infections are usually harmless respiratory or intestinal infections of childhood. Polyomaviruses, remain in the body for the rest of life, i.e. they persist as part of the body microbiome. Upon weakening of cell-mediated immunity they can also become reactivated and cause clinical problems.