If that is not possible take your water from near the center of a lake, not from near shore or from a stream. Although running water in a stream may look cleaner, it is actually more likely to be contaminated. Fill your container from as far under the water surface as possible. Then before drinking follow one of the steps below to purify it. Boiling water for 3-5 minutes is the surest way to destroy the giardia cysts.
Filters are often ineffective even if they say they remove giardia.
If a filter is used, select one with a pore size of 1 micron or less. Charcoal filters are not effective in removing giardia, and some filters that do remove giardia may not remove bacteria and viruses that cause diarrhea. Purification tablets may work if the package specifically states that they are effective against giardia. If you use purification tablets, it is critical that you follow the directions exactly, use enough chemical, and allow the water to stand the required amount of time for complete treatment. Insufficient exposure time will prevent complete elimination of the cysts.