We know, we know — water isn't something edible. Still, it bears discussion under this topic, as thirst can be a powerful motivator. If you didn't carry enough H2O on your hike, don't think you can dip into a pool of stagnant water to quench your thirst. Such water may seem fairly innocuous, especially if it's not covered with a slimy green coating. And certainly taking a long sip from a pool of stagnant water may seem better than going without, especially if it's quite hot out or if you haven't had a drop to drink in a long time. But there are definitely some risks in doing so.
Any pool of stagnant water, or even an area of slow-flowing water, attracts innumerable insects wishing to breed. Top among these are mosquitoes, which are well-known for transmitting serious diseases, such as malaria, dengue and West Nile virus [source: WHO].
Skip the standing water, then, and soldier on until you find a more reliable source of water, like a running stream. Better yet, always make sure to carry plenty of water when you'll be outdoors for a while.