Eli5: How does bacteria form in still water and not running water?
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Bacteria grows in all sorts of water. It’s just that if the water is sitting still, it lets *more* bacteria grow in that tight space and form bigger colonies of harmful things. If the water is running, it’ll all be spread out and there will be less, if any, harmful things in a sample of water.
If you eat one bacterium, it’s likely that it will be destroyed by your body before it can reproduce or make you sick. If you eat 10^12 bacteria, they’ll start reproducing before your body can get a handle on it.
Bacteria are all over the place; you’re routinely eating, breathing, and drinking in bacteria, just not at dangerous levels. Still waters let bacteria grow to high concentrations, dangerous to hour health. Running water is constantly washing the bacteria away, so the concentration in any single unit of water is low.
Running water moves bacteria around. It also keeps a lower temperature than stagnant water. Bacteria benefits from warmer temperatures.
The major thing you’re trying to avoid in water is harmful microorganisms, usually from animal waste and rotting flesh. Moving water will dilute things that fall in and the aeration will help decompose rotting materials. So if a deer shits in a river, you’ll probably be fine. If a deer shits in a pond, it might not be safe for weeks.
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