It's also important to use, store and maintain the filter system properly. Make sure the intake never touches the outlet side. When you are handling the filter system your hands can transfer contaminants in unfiltered water to the filtered side if you are not careful. So just think about what you are doing, be careful and it will do its job just fine.
Al
Thats one of the things I like about the MSR mini-works and other filters that fit directly onto a standard Nalgene bottle. The water goes directly in and all thats left is screwing back on the top. But of course you still have to make certain your hands and the top are clean and dry.