National Health Service (NHS) staff sickness absence in England shows unique on/off switching leading to successive and prolonged periods of higher and lower sickness absence. This on/off switching shows evidence for spatiotemporal spread and the expected stochastic-based variation in the magnitude of the ‘outbreak’ in different locations. Effects due to summer/winter and temperature can be excluded. A unique type of infectious event is proposed. Based on the range of conditions affected the sub-acute effects of the immune modifying virus cytomegalovirus may be involved.