This module is intended to help engineers evaluate the relative health risks from pathogens at land treatment sites versus conventional waste treatment systems. Among the topics considered are the following: (1) the relationship between survival time of pathogens and the chance of disease transmission to humans; (2) the factors that favor survival of pathogens in the soil; (3) survival of pathogens on vegetation; (4) factors which affect the movement of pathogens through the soil and their uptake and retention on soil particles; (5) the possibility of pathogens moving to the groundwater and how long they live in groundwater; (6) the risks of crop contamination and how to minimize them; (7) the effects of various conventional treatment processes on pathogen removal; (8) factors that affect efficiency of chlorination on pathogen removal; (9) ways to minimize disease risks to animals grazing on wastewater-irrigated pasture; (10) insect control at the land site; (11) specific pathogens contained in wastewater; (12) infective doses; and (13) ways to reduce pathogens in aerosols. (Author/BB)