Water that sits for a very long time (years) in a pipe becomes very putrid. There is a different kind of bacteria that thrives in anaerobic situations. The water will smell so bad (like rotten eggs) that the smell can work its way through cheap quality plastic pipe. The bacteria can slowly work its way back into the rest of the water piping during the night when there is little water flowing. Best to run clear water through all pipes at least once every 90 days.