Greater than 90% inactivation as measured by infectivity was achieved by treating oocysts with 1 ppm of ozone (1 mg/liter) for 5 min. This means that C. parvum can be controlled by ozonation, likely true also for E. histolytica, but the evidence is not forthcoming. Suspect tap water is made safe by boiling it for one minute or by passing it through filters of less than 2 microns.