<qa>  What is the unit of randomization and intervention?This study is a randomized, triple-blinded, placebo-controlled, intervention trial in an elderly population i.e. over 55 years of age. The intervention to be tested is household-level treatment of drinking water by joint use of ultraviolet light and filtration devices in the form of a point-of-use, countertop unit. The project site is the city of Sonoma, California, and adjacent regions of southern Sonoma County also served by the Sonoma County Water Agency, that receive their drinking water from ground water aquifers under the Russian River. The unit of randomization and intervention will be individual households. The unit of analysis will be the individual participants within households, adjusted for intra-household clustering i.e. the within-household correlation. The participants, the study investigators and the data analysts will be blinded to the assignment of individual households to active or placebo device except for one study staff member who will keep the randomization codes. This un-blinded staff member will not have contact with any of the study participants, nor will heshe have access to any data on the study participants during the course of the study. 