The Berkey water purification systems can efficiently purify raw untreated pond, lake and river water. However, always use the cleanest and clearest water possible. As an additional precaution, if using a source of water that you believe might contain extreme viral and bacteriological contamination, it is recommended by the CDC, EPA and other organizations that approximately sixteen drops of plain bleach (sodium hypochlorite) or iodine per gallon be added to treat the source water before purifying. This should kill minute pathogens such as viruses, within 30 minutes. The disinfectant will be removed from the treated water entirely with the Berkey system, including any odor or taste. For more information about disinfecting water, please reference the following links. cdc.gov or  water.epa.gov.