GRAYL gives you two filters options. The G3+ filter is faster to plunge and costs $19.95. It gets rid of nasties like bacteria, protozoa, and impurities like certain heavy metals and some industrial and pharmaceutical chemicals. The G3+ Purifier costs $39.95, is slower than the filter, but has two layers of filter media that will remove SARS, hepatitis A, rotovirus, etc., This is definitely worth the investment if you do a lot of international travel, or visit places where water quality is really unknown. Not a bad idea for disaster preparation either.