QUOTE (Draco18s @ Jul 21 2011, 12:23 PM)

"Lack of water" is another issue entirely.
"Lack of clean water" has to do with the fact that small-scale cleaning (enough to keep yourself hydrated in the event of the apocalypse) isn't sufficient in the long term1, materials may be hard to come by, and do require some level of education to construct. Large-scale cleaning is out of their ability as a larger economy.
1The method I've mentioned is very basic and you should still boil your water. In order to get "even better" you need pressurized sand, which has to be back-washed daily in order to stay viable. Clean water at the level that we have it in the developed world is only 200 years old, or so (and the problem has only existed as a scientific one since the early 1600s).
Poor sanitation as well as socioeconomic issues are also responsible for the lack of clean water in many of the areas suffering as such. Regional power struggles and/or lack of regulations on certain resources or the processing of waste all result in the inability to construct, maintain, and benefit from large-scale water purification and aqueducts, not to mention the other resources (advanced education, medicine, etc.) that such problems limit in those areas.
Edit: There is one method for water purification that works on a daily basis, and requires only the technology to work with glass well enough to create a greenhouse. Construct a decently sealed glass Pyramid with water poured into a container in the middle. The sun will evaporate the water in the middle container, causing it to later condense on the outside walls and flow down to the outside of the pyramid. The water on the outside should be very pure, at the very least clean enough for personal use. However, this goes back to those problems listed earlier