Sawyer has a gravity filter in the $200-300 range that is effective on bacteria and viruses. They have both bladder style and bucket (5 gal.) systems. I bought the bladder style, but haven't had a chance to check it out. Guaranteed to treat 1 million gallons of H2O! Field servicable and extremely portable. Supposed to filter 2 liters in about 15 minutes via gravity. This seemed like a good thing to have on hand to filter collected rainwater, riverwater, etc. in the event that public water systems are out of commission for any reason.