Two to six weeks: that’s how long the excessive diarrhea will last if you’ve ingested giardia, the most common water-born parasite in the world. Ninety percent of America’s surface water is contaminated with giardia, but you can only survive three days without water. Most commercial purifiers kill giardia and other micro-organisms found in mountain streams and lakes, but what if you’re suck in the middle of the woods without your fancy gear? What if you lost your pack? What if you’re desperate for water but afraid to sip from a stream because of the potentially explosive after-effects? Here’s a step by step process to filling one of man’s most primal necessities-purifying water naturally-without falling prey to giardia, aka “beaver fever.”

1) Know the Source: “Don’t get water from a standing pond or running creek,” says Reggie Bennet, operator of Mountain Shepherd, a primitive skills camp near Amherst, Virginia. “Animals and livestock defecate near these sources and contaminate them.” However, not all running water is contaminated, especially at higher elevations. Generally the higher you go, the fewer contaminants.

2) Make a Filter: Natural springs are usually free of organisms like giardia, so all you need to do is filter the water for particles. Build a homemade filter with sand, moss, grass, small pebbles, and if you can find it, burnt wood crushed into fine charcoal. Cut off your pants leg and layer the items inside the tube of cloth, tying the bottom into a knot. Pass the water through the pants leg and collect it out the bottom. The first few runs will be murky, but the water should clear up.

3) Get Primal: “If you can’t find a natural spring, collect water from a stream in a plastic bag,” says Bennet. Sandwich bags or garbage bags will work. Seal the bag and let the water sit in direct sunlight for three to four hours. The UV rays will kill most viruses and bacteria that you have to worry about.

4) Choose the Lesser of Two Evils: Getting sick beats dying, hands down. If you can’t find a spring, lost all your zip-lock bags, can’t build a fire to boil water, and can’t make a homemade filter, drink anyway. Suffering from weeks of diarrhea is better than dying from dehydration.

State of the Art

Despite all of the fancy filters on the market, it turns out good old fashioned sunlight is still the best way to purify water. The new STERIpen harnesses the power of the sun and uses concentrated UV light to kill 99.99% of dangerous bacteria and viruses. Just make sure you don’t lose it.