Cysts are the one celled, egg-like phases of some protozoans. Some are found in water and while a number are harmless, others such as Giardia and Cryptosporidium cysts can lead to disease. It's difficult to kill these cysts because they're protected by a tough outer membrane that resists even chlorine bleach. Special Activated Carbon filters can be installed at home to block them from entering your drinking water supply.
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