don't take the following as "all inclusive". i could be forgetting something important. as i've stated elsewhere it's been 30yrs since i've worked in this field. i'm making this post long in an attempt to answer any additional questions that might come up. hope i don't confuse the issue further or put anyone to sleep.
i should further note that 4yrs ago i had an accident that left me with permanent physical and cognitive deficiencies (memory being one of them) cp. to my pre-accident state. without going into detail, my memory used to be quite good. now, my memory is not, euphemistically, what it used to be 4yrs ago. if i'm not certain of something, or think that i may not be recalling something correctly, i will say so. however, that said, i'm not always certain that i may not be recalling something accurately, so it's best to check a more authoritative source.
1. even boiling, at sea level, for a reasonable period of time won't kill all pathogens/parasites. this is why sterilization in laboratories and hospitals is done, typically (30yrs ago) using one of two diff. methods depending upon the items being sterilized. 15min at 15psi above atmospheric pressure (water boils at 250deg F at this pressure) will kill anything - this is commonly done in a hot steam autoclave. some type of "ethylene" gas (as i recall - can't really remember precisely what gas) is used for gauze, towels, etc (basically, anything that should not be exposed to high temps or moisture). on the other hand, since milk can't survive high temps (casein, lactose, and other chemicals in the milk would be affected), milk is commonly pasteurized (some little buggers survive in very low numbers) using one of two methods - HTST (hi temp short time) which is (i hope i recall this accurately) 180 deg F for about 15 or 20 seconds, or LTLT (lo temp long time) - i think this is about 160 deg F for a couple of minutes (i'm really forgetting this one, so you should check it out). i only mention these facts so that a fuller picture is given of what it takes to kill "germs" using temperature. pastuerization is used on milk so that it doesn't spoil as quickly (and to kill some enteric, or found in the gut, pathogens which are more sensitive to heat) that is supposedly already safe and free from pathogens due to antibiotics and clean, non-contaminated milking methods. so, pasteurization techniques should not be interpreted as being appropriate for sterilization and killing of all pathogens.
2. i said pathogens - most (not all) which are NOT normally encountered in water. that's the good news. these pathogens normally inhabit soil (or infected cattle & humans - this will become clearer in a moment).
3. those that you might encounter in water, viz. some eggs of worms and some protozoans (again, there might be others, even in No. Amer. that i'm forgetting) can be killed by boiling, but you need 10 minutes (to be safe) of a rolling boil at sea level boiling temps. obviously, higher elevations will require more time at a rolling boil (BUT the less likely you are to encounter any pathogens/parasites - generally speaking). so, in a survival/emergency situation, you could do so - it's much too fuel inefficient for normal purification means. it's NOT enough to simply bring the water to a boil. a rolling boil should be maintained for at least 10minutes. how many really do this however? 10min. is a long time!!
4. now, what are the little buggers that won't be killed by boiling - for any reasonable period of time, if at all? these are primarily soil bacteria, though it's possible (though very unlikely) that they can be encountered in water (diseased cow dies on the edge of a barely moving body of water; it's legs are sticking up out of the water and so is part of it's bloated body - but it goes unnoticed as do the vultures circling overhead and the ravens picking at the carcass?!!! i hope you get the point i'm making here). mainly two related genuses of spore forming bacteria. three anaerobic species and one aerobic cousin. these are some of the largest non-spirochete (non spiral shaped bacteria like those causing syphillus & lyme's disease) bacteria. as i recall, they are over 1.5microns in length, sometimes 2 or 2.5 microns in length. they are over 0.5 microns in diameter. they are all bacilli, or rod/cylindrical shaped bacteria with rounded ends. all stain "gram-positive" and so have ~80% mucopeptides in their cell wall and therefore are generally quite susceptible to penicillin or penicillin -type/-derived antibiotics (inhibibit cell-wall synthesis in "gram-positive" bacteria due to the high mucopeptide content), as well as some newer types of anti-biotics i'm not very familiar with.
Clostridium botulinum (causes botulism) [i should add here that there is a common misconception about "simmering" neutral pH soups, stews, etc - French Potato Soup for instance. The injunction to "simmer" (not quite a rolling boil) such food stuffs for 10minutes is NOT because it will kill any C. botulinum spores. This instruction is to "denature" the therm-labile (heat sensitive) botulism exotoxin that may be present (remember potatoes grow in the soil and may not have been adequately cleaned). Any spores still present will be ingested, will germinate in the lumen of the gut and can cause varying degrees of problems, from cramping and gas to worse, depending upon the "load" or amount ingested.]
Clostridium tetani causes tetanus, "lock jaw" - found in soil, NOT rust - however if you step on a nail, it is usually on the ground in contact with soil [unless you're a "roofer"] - puncture wounds are the concern here, NOT lacerations. punctures often heal at the top first, leaving a void beneath - C. tetani is anaerobic & loves this enviroment since oxygen is rapidly depleted from it - any type of wound that heals from the bottom to top is generally NOT a candidate for C. tetani colonization.
Clostridium perfringens (causes gas gangrene aka gangrene - very nasty visual & olfactory effects - seen & smelled it, not pleasant - even less so for the person infected)
Bacillus anthracis causes anthrax, nasty disease (two main forms of the disease) - but terrorism has taught us all about this bug. even this guy, 30yrs ago, was cultured in college micro. classes by undergraduates - it just wasn't inhaled, injected under the skin, eaten, aerosolized, or turned into so-called "weapons" grade anthrax. in fact, to prevent theft, used to keep my lunch in the biohazard refrigerator having many cultures of pathogenic bacteria - all safely in "stoppered" test tubes (rules weren't followed as strictly then as they are now). maybe this accounts for how i turned out??? sure explains alot!!!
these bacteria form endospores when they encounter an inhospitable environment. this protects them until they can find a nice, cozy environment. as far as i know, it's extremely unlikely that anyone will contract these from drinking water. the worm eggs are more likely (and still very rare apparently). i've only mentioned the "boiling" issue prev. for two reasons: 1) to correct a common misconception that "boiling" is to "bugs" as kryptonite is to Superman - not exactly true., and 2) if one understands more fully the limitations and consequences, then one can make a more informed choice/decision.
am i concerned at all about any of these four bacteria in water in the wilds? no. but i'm not going to throw a bunch of nice dark soil (diff from sand) into the water and then bring it to a boil and drink it, nor am i going to drink any water that has a bloated cow in it. beyond that, my prev. comments were more for being precise (dotting every "i" and crossing every "t" - it's an anal-retentive, Type A, OCD fault of mine - drives my wife crazy - she's a saint, so she's put up with it for 26yrs, and it's only gettin' worse as senility sets in).
all of the above bugs should be easily filtered out using a 0.2micron absolute pore size filter, or in some cases, depending upon filter design, a sub-micron nominal pore size filter employing a labyrinth design (this is essentially how natural soil filtration works).
for my part: i generally use just AqM for purifying water. if i were really concerned about a water source, then filter (using a 0.2micron ceramic filter) & either chem. treat using AqM or UV-C.
hope i didn't upset anyone previously. if this fails to ans. you questions, please post back & i'll try to be briefer and clearer in my answer to more specific questions. being responsible just for myself and posting advice/info for others are two diff. things in my mind, requiring diff. levels of "preciseness" and caution - hence this long post.
i'm about at the point of ending any kind of technical post on this and other subjects which i'm very interested in. i need to remember not everyone has the same level of interest in the same subjects i'm interested in (be warned: if you think this post is long, don't ever get me started on Auto Sports, performance driving skills, performance modifications to street cars, or auto repair - not that these have any place on a BackPacking website.). furthermore, i'm thinking that i cause more problems and raise more questions than i answer.
i really wish a medical doctor, or an experienced microbiologist with more recent, more thorough knowledge would chime in here and help out. is there a doctor in the house?