Bacteria are small living organisms that can be found in pools, although a pool that is well sanitized should consistently keep bacteria counts below unsafe levels.
What is are Bacteria?
Bacteria are usually single celled organisms commonly found in swimming pools. Bacteria levels in pools are are controlled by using sanitizers, by chemicals, specialized filter cartridges, devices such as UV (ultraviolet) emitters, or any combination of systems and chemicals. If the sanitizing ability of the pool is inadequate, bacteria can rapidly grow and reach levels in the pool water that would be unsafe for swimmers.
How can Bacteria affect pool swimmers?
Maryland Department of Health & Mental Hygiene, http://ideha.dhmh.md.gov/IDEHASharedDocuments/swimming_related_illness.pdf