Jon look at the center for disease control web site contact your local aids project become aware . HIV is very sensitive to temp it doesnt live long outside of the body . HIV in human waste is so tiny that its not a concern and does die away from body temps . And you certainly should not be concerned about waste water that sits in a septic tank for long periods of time , if that septic system is working properly it will allow ALL waste to break down in the tank as it is diluted the bacterial action in the tank should be rendering everything into a rather harmless liquid with no other effect . HIV IS transmitted primarily from UNsafe sex and shared needles . BTW your nieghbor is under no Obligation to tell you anything about there medical history . You have more to fear from neighboring propertys using pesticides ,herbicides , or doing other things that could be far more harmfull to your areas water table If your all on private wells .