Water ranks just below oxygen in the list of elements that are essential to human life. When their home consisted of an outdoor camp in season one, boiling water was a regular on-camera survivor chore. And it's hard to erase the image of a walker being fished out of a well in season two (only to end up with his lower torso dropping back into the water). Yet not a lot of attention has been paid to where, in fact, the survivors are finding clean water–or whether they're drinking it at all. "All water should be treated for biological contamination if possible," says Dessinger. "Bringing the water to a rolling boil is the most effective way to treat it." The best way to determine whether a water supply is clean in the first place? "I would be suspicious of a persistent body of water (a pond, not a new rain puddle) that was devoid of life," says Dessinger. "If there are not insect larva or algae in the water, it may be toxic. Mosquito larva in your water is good – just think of it as extra protein."