most bacteria takes a wile before they affect you, but personally i prefere to drink biological safe water. i even boil tab water first at home ( habit i got from my parents ). Purfication tabs doesnt take much space and gives you less to concern about ( the phycological stuff ) in a survival situation, if you get stranded for long periodes you wont be able to treat diseases caused by some waterborn bacteria. and yes purfication water will run out if you have to survive for longer periodes, but most of use got alluminium foil with's can boil water to make it biological save.....