Thanks for the reply and description.
I understand now what your are referring to. When you said "check valve" i was picturing a valve designed solely to prevent backflow in a fashion similar to what is used in some larger hydraulic systems, e.g., some submarine nuclear reactor systems. These you don't turn on and off. They operate automagically to prevent backflow and do nothing to prevent flow in the normal/forward direction. Hence my confusion. Thanks again for the explanation.
As far as the filter. By 'fold' i was referring to the Crypto 'spore', not the filter.
As for pore size, check even the BPL on-line store. It's a 2micron pore size, not a 0.2micron pore size. Hence, most bacteria will pass, including E. coli, which is an indicator of fecal contamination and only rarely are certain strains/sero-types considered pathogens, as well as many true human pathogens will pass through the filter as well. However, in most cases, hikers are NOT as concerned about bacteria in backcountry water sources as they are about spores/cysts.
As far as 'spores' go, the BPL store's webpage is largely correct. Most Crypto will be filtered out, but there is an uncommon (i hesitate to use the word rare in this case) occurence with Crypto spores where they will fold over on themselves and thus be less than 2microns in any dimension. As i said, i have observed this myself viewing C. parvum under a microscope several decades ago. These may pass through the Seychelle filter purely from a size/geometry standpoint. The Seychelle appears to be a labyrinth type of mechanical barrier filter, so this, IMHO, will reduce the likelihood of this happening, but it simply can't be ruled out, IMHO.
How bad is C. parvum? Varies by the person and the strain - some more virulent than others. BTW, it's estimated based upon samplings taken from various geographical areas that 85% of the NA population has been exposed to Crypto at some time in their life. I think most people are more concerned about Giardia than Crypto, particularly Thru-Hikers (due to the onset of each after ingestion - Crypto typically about a week, but can be as short as a couple of days [the virulence of Crypto varies widely as well as some people's tolerance for it] or longer in some cases; Giardia is typically 1-2weeks). My point here is that many hikers will be home before they experience the symptoms of Giardia (that's NOT to say that this is 'ok', only better - relatively speaking - personally, i'd rather be near the porcelin altar than out on the trail), whereas in some cases Crypto may hit the short-Trek hiker while still on the trail. Also, i believe in most cases Giardia may produce a worse effect. My wife and i have both had Giardiasis - many yrs ago. Fortunately, we were both home by the time the symptoms appeared. Unfortunately, from my wife's perspective, she had it worse; though mine was no pleasant experience by any means either.
I hope that this clears up what i was trying to say in my prev. Post. Sorry, i wasn't clearer there.
Thanks again for your reply and detailed description of the 'check valve' and gravity filter. Makes sense to me now.