The concept of cost has not, until recently, been investigated in any detail within the framework of epidemic theory. In the case of minor illnesses a pertinent estimate of cost may be the man-hours lost in industry during an outbreak. For the general epidemic, in which the removals are immune recoveries (or deaths), this is conveniently represented by the area generated by the trajectory in time t of infectives I(t) until the epidemic ceases. A small element of area under the trajectory is I(t)δt and represents I(t) infectives withdrawn from the labour force for a small time δt; the total work-time lost is thus represented by where T is the duration of the epidemic.