Beaver Fever

Don't drink from water sources which contain beavers. You could easily contract beaver fever which has been known to be potentially deadly.

If you're in real desperate need for water and you happen to find a river with beavers, try to head upstream as far as possible.

Either way, beaver fever symptoms only develop after 9-15 days, so it's worth the risk to drink contaminated water since it'll enable you to survive a few extra days.