I "ditto" Doc Ron's position of not worrying too much about the bad boy's found in water... under the circumstances presented in this exercise. 4-5 days seems an ample time period to locate a helpful nurse and ask to be admitted into her domain(hospital), if necessary. The body does build immunities! I am interested in reading Hoodoo's references, and I will. Thanks.
Without specific experience along these lines, here is what I would do with the material's available. Material's available: clothes on my body, shoes, belt holding up my pants, knife, Kydex/plastic sheath with a hole in the bottom( I allow this since it is what I carry),a fire starter. Nothing else. ( thank's Sgt.Mike). I would start with a very,very strong positive attitued, under the circumstances. I have been in a situation similar to this in extremely cold weather. I did have more materials.
The shelter has been built, the fire is underway. Place a handfull of sand on a flat thin rock, both of which were gathered a little above the water edge at the stream. Place the rock with the sand on top into the fire's edge(level spot). Sanitize the sand( get it extremely hot). Any bad boys living in the sand will die the same as they will in boiling water. They Must have a moist enviroment to live in. Take a small log out of the fire...one that has burned to a charcoal stage. Let it cool,then wipe it down to get off the surface material. Crush the charcoal into small pieces, small enough to fit into the sheath. Mix the sand and charcoal together and place in the sheath. Pack firm but not tight using a flat long stick. At the water's edge, get a handfull of mud or clay ( whatever is available), squeese as much water out as possible( it will still be wet ). Place this substance into that shallow hole you fashioned into the sand over there..looks like a bowl impression. Press it into place, outlining the impression. The beauty of frigid weather is that everything freezes nice and tight ( including your hands). Let the mud/clay bowl freeze. Warm your hands at the fire station and then take the sheath( now a make-shift filter of sanitized sand and charcoal) to the water's edge. Sing a sexy song, keep a high attitued and get ready to drink some wet water. Take the frozen bowl out of the sand, dip it into the water. Poor the water into the sheath, place the bowl under the sheath, just below the hole in the sheath and hang on. While the water is filtering through, remind yourself that Betty crocker would be very proud of you!
Sand, charcoal, water, sheath,a knife, frigid weather, adequate shelter, warm fire, positive attitude. Will my filtration system work? I have no idea. But if filtration is paramount to drinking or not drinding water, I would try this knowing that if it failed, I could at least get to bed, say a little prayer that it does not go below -15f and try to have an enjoyable " wet dream." I am ready to learn, anyone else out there have ideas or real life experience? Go!