Boiling your water before drinking it is sound advice. Just getting the water to a boil will kill off most nasty stuff, including Giardia. However there are hardier species of viruses and bacteria that can survive even at high temperatures. To be safe make sure you boil your water for at least five minutes. Boiling water isn't just good advice, it is also the cheapest way to remove biological pathogens from water. One thing that boiling will not do is remove chemical contaminates. If your water source is cloudy and or devoid of any life (fish, plants, algae, frogs or other amphibians, etc.) it might not be safe to drink regardless of how long you boil it.
Chemical treatments have been around for most of the twentieth century in one form of another. People that live in hurricane prone areas are told to keep some chlorine bleach around as a water purifying agent, a small amount of chlorine bleach per gallon can kill off most pathogens without making you ill IF you let it sit long enough. Most store bought solutions are based on iodine and/or chlorine tablets. Tablets are dropped into the water and are allowed to sit for at least 30 minutes. The iodine kills off the pathogens while leaving the water potable, well sort off. The iodine will give the water a funny taste, best described as eating a band-aid. Some treatments have ascorbic acid (vitamin C) tablets to treat the treated water, and remove the iodine taste and replace it with a slightly tart aftertaste. The main issue with chemical treatments is the water needs to be warm, at least 70 degrees to be effective. Cold water may need treatment all night to be safe. People who have thyroid problems can have serious medical issues with ingesting iodine, and prolonged exposure to iodine has been known to cause thyroid problems.
This leads to the third solution, which is filtering. Filters come in all sorts of shapes, sizes, and capacities (we could write a whole story just on filters). Filters can offer three different options, a simple microbiological filter with pores measured in microns, a carbon filter system as well as pores, and/or an iodine filtering system as well as the pores. If you have a filter or are looking to buy one, don't waste your money on anything that has filter pores larger than 0.2 microns. Anything larger and bacteria can still pass through, however the much larger Giardia cysts and protozoa will still be filtered out. A filter without iodine will not remove viruses, so in the end you may have to boil your water anyway or treat with iodine. A filter with carbon or charcoal has a major advantage over the other purifying methods; it can remove dangerous chemicals from the water that tablets and boiling will leave behind. Another benefit of filtering is the water will usually taste better.
The final method of filtering that is at least worth mentioning is reverse osmosis. Very expensive, complicated, bulky and heavy, reverse osmosis filters are not practical for camping. They have the benefit of removing just about anything nasty from water including, viruses, bacteria, amoeba, protozoa, and chemicals. A reverse osmosis filter can even desalinate seawater.
One thing to keep in mind if you are boiling, using tablets, and not using a charcoal filter is they will not take chemical contaminates out of the water. Arsenic, lead, and other heavy metals, dangerous amounts of potassium, phosphorus, and heavy sediments will all be left behind. In certain places like Badlands National Park in South Dakota no amount of treatment, short of dragging a reverse osmosis plant with you is going to make the water potable. In areas where there is a lot of agriculture activity and leeching from fertilizers this can be especially dangerous. The bottom line, know your water source as best as you can.
This leads us back to Giardia lamblia...
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Canoe
Over the last 25 years a trend has grown through the backcountry of the United States. Giardia lamblia, an amoeba like protozoa has been spreading into streams, rivers and lakes. Found in two forms, both as an active trophozoite and as a dormant cyst, Giardia is responsible for making tens of thousands of people sick each year in the United States. Although infection by drinking untreated water is one of the most common causes of Giardiasis, Giardia is a growing health hazard in our treated water supplies and is reaching an epidemic level in day care centers.