Public health activity is costed at the program level, 11 and effectiveness and other measures are estimated at the aggregate level as theoretical constructs (e.g. population health status, potentially avoidable mortality). It is difficult to tell when public health effort and investment is effective, even over long periods of time; the small amount of work to this end is bedevilled by the poor quality of available data, the complexity of costing public health activity, 12 and lack of agreement about what should be included. 13 Costs to society when public health fails (e.g. cryptosporidium outbreak response, effect of SARS panic) may be easier to estimate.