Polyomaviridae is a family of viruses whose natural hosts are primarily mammals and birds. 13 species are known to infect humans, while others, such as Simian Virus 40, have been identified in humans to a lesser extent. Most of these viruses are very common and typically asymptomatic in most human populations studied.
All the polyomaviruses are highly common childhood and young adult infections. Most of these infections appear to cause little or no symptoms. These viruses are probably lifelong persistent among almost all adults. Diseases caused by human polyomavirus infections are most common among immunocompromised people.
As with many recently discovered human polyomaviruses, the clinical significance of HPyV9 is poorly characterized. Attempts to detect polyomaviruses in a variety of tumor types have consistently found that HPyV9 is either absent or present at very low viral loads, indicating that it is unlikely to be causally related to the tumor.