Case clustering in pityriasis rosea: support for role of an infective agent.

One hundred and twenty-six cases of pityriasis rosea seen over two years in north Staffordshire were analysed for clustering in time and space. A statistically significant degree of clustering was found; this was evident among female patients considered separately but not among male patients. The incidence of the condition was higher in patients working in, or attending, educational establishments. These findings support the hypothesis that pityriasis rosea is caused by an infective agent. A search for an infective organism and a transmission mechanism now seems justifiable.