1. Boiling is the surest way to kill dangerous organisms, but some hikers feel it is impractical because it uses fuel and takes time. Bringing the water to a rolling boil kills harmful organisms, including Giardia cysts.
2. Iodine tablets are lightweight and simple to use. Iodine makes the water taste foul, so carry fruit-flavored crystals or powder to make the water drinkable. Tablets have a limited life, so buy a fresh supply at least annually. Once a bottle is opened, us it within a few weeks or else discard it. Iodine does not remove the parasite Cryptosporidium. Despite what is often stated in outdoor literature, iodine is not highly toxic. If you digest too much you'll probably vomit, getting rid of most of the iodine in the process. Normal doses won't harm you.
3. Filtration is the most popular method of water treatment. Filters can remove bacteria, organic chemicals, and protozoa, including Giardia. They do not remove viruses unless they also include chemical disinfection, in which case you might as well just use iodine. Viruses are the most dangerous waterborne organisms, but viruses aren't a problem in most of North America. If you visit the Third World, they are a real threat.
   • Filters all clog up at some point, and some backpackers feel they are too heavy, too complicated, and too inefficient for backpacking. Filters sometimes clog or break.
   • Weights of filters range from 6 ozs.-13 lbs. (for large expeditions). Those designed for typical backpacking run in the 8-24 oz. range.

The protozoa Giardia lamblia is the most well-known parasite to North American hikers and backpackers. This protozoa, which causes a virulent stomach disorder - giardiasis, or giardia - curable only by specific antibiotics, has received much attention in the past few decades. A fear of giardia swept through the backpacking world in the 1980s. While the parasite that causes giardia is indeed found in many wilderness streams and lakes, many people are probably too concerned about it. Giardia isn't fatal, although you may become incapacitated from the associated dehydration it if you become ill far from help. While some people experience violent stomach illness, others will hardly notice any effects.

Giardia lamblia lives in the intestines of humans and animals. It gets into water as cysts excreted with feces, which is one reason always to site toilets well away from water. The symptoms of giardia appear a few weeks after ingestion, and include diarrhea, stomachache, a bloated feeling, nausea, and foul-smelling feces. However, theses symptoms occur in other stomach disorders as well, and only a stool analysis can confirm infection.

The chances of catching giardiasis aren't that high, however, despite media coverage to the contrary. The understandable desire of park and forest agencies to avoid litigation adds to worries - they generally advise people that all water needs treating.