Access to a safe, drinkable water supply after a disaster is critical. You can't survive more than a few days without water, and adults need at least 2 quarts of water every single day. So if you could only have one piece of prepper gear to help you survive a disaster - that item should be something to get you water to drink.
If the water taps stop working, people will be forced to find water from nearby sources. But water from fresh water sources such as lakes, streams and rivers contain various bacteria, protozoan, viruses and other parasites. Some of the most common bacterial agents that can make you sick include E Coli, Dysentery, Cholera and Botulism. Protozoan includes species such as Cryptosporidium Parvum, and Giardia Lamblia. And then there are larger creatures that can contaminate water such as tapeworms of various sizes. Some of these could be lethal even with hospital care!
At best, drinking contaminated water containing any these microorganisms can cause diarrhea and vomiting. In a survival situation, food and water are valuable resources to be conserved as much as possible. But should you get diarrhea and start vomiting, that drink of contaminated water would be counter-productive. You'll need to eat even more food to replace what you've lost, and instead of that drink hydrating you will actually become dehydrated! All on top of feeling horrible!
But contaminated water can be made safe for drinking using a number of methods. Water can be filtered, chemically purified, purified with UV light, boiled, distilled or some combination of these methods. Note: Be careful when handling potentially unsafe water - a splash on the skin, or in the eye could also make you sick.