A wide variety of viruses are found in raw wastewater. Although a large number of these viruses are inactivated during aerobic wastewater treatment, the surviving viruses are likely to be associated with the solids produced during the wastewater treatment. The large number of viruses excreted by infected individuals may produce a relatively high virus load in raw sewage. Aerobic digestion of sludge is accomplished by rapidly stirring sludge to promote aeration or by forcing air into the sludge. Anaerobic digesters are designed to exclude oxygen and a fermentative digestion of sludge occurs in contrast to the oxidative digestion that occurs in aerobic sludge digestion. Although quantitative studies on survival of viruses during aerobic digestion of sludge in full-scale aerobic digesters are not complete, it appears that the virus level is reduced during aerobic digestion of wasted sludge. Physical treatment of sludge such as drying, heating, and irradiation reduces the numbers of viruses in sludge to undetectable levels.