Pond, river, lake, or rain water often contain a harmful host of bacteria and viruses that would like nothing more than to make you feel like you're dying. Of course, if you're camping at a national park or trying to make it through the worst of a natural disaster, those sources might be the only way to get fresh water. Fortunately, a camping filter or emergency water purification system can clean up even the dirtiest water, so you can drink safely without having to worry about getting sick.