Epidemiology, microbiology, and risk assessment of waterborne pathogens including Cryptosporidium. 
Cryptosporidium is one of a suite of relatively recently emerging pathogens of concern in drinking water.  Based on human dose-response tests, guidelines for exposure yielding defined levels of endemic risk have been developed.  This risk assessment procedure is grounded in the process used for chemical risk assessment.  From outbreak data, critical concentrations in water that may lead to epidemic levels have been postulated.  Development of these levels will be discussed.  Validation of the information using outbreak reports from the 1993 Milwaukee incident can be made.  Use of this approach must be tempered by the existence of substantial waterborne cases in the absence of detectable oocyst levels as in the Las Vegas outbreak, and (apparent) high levels of oocysts without (apparent) significant health effects as in the case of the (at the time of this writing) ongoing incident in Sydney, Australia.