A departed friend of mine, Jon Tuttle, told me about his uncomfortable experience with beaver fever (giardiasis). From that point on, he was very careful to filter his water on backpacking trips -- unless he was getting water from melting snow on steep hillsides.
Beaver trivia: Beaver testicles were used as a pain reliever by native tribes. The beaver transforms salicin (from willow tree bark) into salicylic acid, which acts like aspirin! Beaver testicles were an indian trade item -- but please don't leave any in this cache!
Look for something the size of a 35mm film container. Log only. BYOP.