What causes pityriasis rosea?

Research is yet to discover a medical cause for pityriasis rosea. Fungi and bacteria have been ruled out as causes of the disease, and it is well known in medical circles that the disease is not an allergy. A virus is suspected of being the cause, but there is yet to be adequate research to prove this assumption. Also, that pityriasis rosea is not contagious, and cannot be passed from a diseased person makes the chances of a virus being the cause even less doubtful.