Roger Caffin writes: "But more importantly the presence of a small amount of dirt or organic matter in the water will not greatly affect the operation of UV light." However, Natick labs gave a bad review of the steripen because it doesn't work in turbid water:
I have said this before, but I will reiterate. The primary threats to hikers from water are (a) poisonous chemicals, such as AS (the spell checker says that the metal with this chemical symbol is profanity) or the toxins released by algae blooms (b) helminthic parasites. A healthy adult with a normal immune system will suffer no more than minor distress from the protozoa or virus found in North America or Europe. Some type of of bacteria from in North America and Europe, such as particularly nasty s trains of e.coli might kill a child, but they are unlikely to kill an adult. All viruses, bacteria and protozoa found in North America and Europe can be shaken out of the body with relative ease and pose no long term danger to a healthy adult.
Chemicals can kill, but these are a minor consideration other than in the desert.
Helminthes, on the other hand, are a real danger. Anywhere there are canines (dogs, wolves, foxes, coyotes) there is the potential for hydatid cysts from echinococcus granulosus tapeworm. This can cause very severe illnesses. Currently, this tapeworm is common in all arctic regions (Alaska, Canada, Siberia) where it cycles between wolves and caribou/deer and similar prey. This tapeworm is also very common in the Alps, where it is carried by a combination of foxes and small mammals such as mice and voles. This tapeworm is/was very common in many sheep-rearing areas, such as Iceland, New Zealand, Australia, the entire Mediterranean area, Patagonia, where it is carried by a combination of dogs and sheep. The cycle is completed when dogs are allowed to eat the entrails of slaughtered sheep. This is not done in the United States or most of Northern Europe, and hence the disease is not common there. But there is nothing that says these worms couldn't be introduced and then be perpetuated by coyotes/wolves eating raw sheep. Supposedly these worms have been eradicated recently in New Zealand by a long campaign of veterinary care of the sheep.
Anyway, if you are hiking in any part of the world where there are canines, meaning all of North America and Europe, there is the potential for very dangerous helminthes in the water. Perhaps not now, but these worms could easily be introduced accidentally, such as by wolves migrating down from the Arctic into temperate North America.
Hydatid cysts are a major health problem in the poor sheep-rearing countries of the world, such as Turkey. The cycle is as follows. Unvaccinated sheep-dogs are carriers of the worms, the worm eggs come out in the dogs intestinal output (I tried to use the technical word, but the spell checker says this is profanity), little children playing around the dogs get the eggs in their mouths, the eggs hatch and burrow into the body where they form cysts. These cysts grow slowly and after a dealy of about twenty to thirty years, the person has major health problems. Treatment is quite risky, especially if the cyst is in the brain or heart.
I have been unable to find information as to the probability of ingesting worm eggs in water. It is certainly possible and the National Park Service in the Lake Superior area warns about these worms. But perhaps this is just cover-your-ass behavior on their part. I suspect the likelihood is much greater than commonly thought, and the reason we don't hear more about these worms is that there simply aren't that many people in the backcountry and the damage take 20-30 years to develop and it takes a trained physician to even recognize that the damage is due to worm and not something else. In Turkey, where hydatid cysts are common, physicians knows to suspect worms when someone in their 30's shows up with all sorts of strange symtoms. I'm not sure if doctors elsewhere are so informed.
Worm eggs are ultra-tough. Ultraviolet light has no effect on them and neither do chlorine, chlorine-dioxide or iodine. The only way to avoid ingesting worm eggs is to filter the water. Luckily, the worm eggs are fairly large, like 15 microns and up, so anything which screens for giardia will also screen worm eggs.
There's tons of information on the internet about worms, but very little about the probability of encountering such worms in places like Sierras or Appalachians. However, there are wolves, foxes and/or coyotes in these places, and thus there is the potential for a problem.
Echinoccous granulosus is just one type of helminthic parasite, but probably the biggest risk for backpackers. There are many other helminthes, all of which produce eggs which are fairly large but immune to UV and chemical treatment.