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Stagnant or Stailment water can be dangerous for drinking because it provides a better incubator than running water for many kinds of bacteria and parasites. Stagnant water is often contaminated with human and animal feces, particularly in deserts or other areas of low rain.
Causes and prevention 
Stagnant water may be classified into the following basic, although overlapping, types:
- Water body stagnation: stagnation in swamps, lakes, lagoons, rivers, etc.
- Surface and ground water stagnation; puddles
- Trapped water stagnation. The water may be trapped in human artifacts (discarded cans, plant pots, tires, dug-outs, roofs, etc.), as well as in natural containers, such as hollow tree trunks, leaf sheaths, etc.
To avoid ground and surface water stagnation, drainage of surface and subsoil is advised. Areas with a shallow water table are more susceptible to ground water stagnation due to the lower availability of natural soil drainage.
Excessive watering may cause ground or surface water stagnation.
Life that may thrive in stagnant water 
Some plants prefer flowing water, while others, such as lotuses, prefer stagnant water.
Various anaerobic bacteria are commonly found in stagnant water. For this reason, pools of stagnant water have historically been used in processing hemp and some other fiber crops, as well as linden bark used for making bast shoes. Several weeks of soaking makes bast fibers easily separable due to bacterial and fermentative processes known as retting.
- Lepisosteidae (gar)
- Northern snakehead fish
- Pygmy gourami
- Siamese fighting fish
- Spotted barb
- Walking catfish
Stagnant water is the favorite breeding ground for a number of insects.