WU polyomavirus (also known as WU virus, WUPyV, or Human polyomavirus 4) is a virus of the family Polyomaviridae. It was discovered in 2007 in samples of human respiratory secretions, originally from a child patient in Australia who presented with clinical signs of pneumonia and in whom other common respiratory viruses were not detected. Follow-up studies identified the presence of WU virus in respiratory secretion samples from patients in Australia and the United States, suggesting that, like other human polyomaviruses, WU virus is widely distributed.