Why does untreated lake and river water not make us ill each time we swim in it?

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Swimming pools require mass amounts of chemicals and treatment to ensure it’s safe for swimming, but why not natural bodies of water? Given the amount of waste and bacteria, coupled with water creeping into the ears, nose, etc., why don’t we see more cases of infection?

In: Biology
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you can get sick from lake and river water. usually it’s not the case tho. the bacteria in natural bodies of water isn’t usually very strong and your immune system can kill it quite easily.

chlorine in pool water is there to stop alge from forming in the still water and to kill bacteria that you shed off your body when you swim.

Standing water can be very dangerous, regardless of it being natural or not. Pools in general can be very unhygienic due to humans being humans and losing bandaids, peeing, etc.

But standing water in nature is not great for you, either. However, like most bad things, it’s much worse if ingested.

There are bacteria and parasites in lakes and rivers, but usually not in enough concentration that it be dangerous to be around. However, it’s still unwise to drink from an untreated water source without first sterilizing it. Here’s a few reasons how they are different from your pool:

1) Rivers are, generally, constantly flowing. This constant flow keeps most dangerous things moving, so they can’t cluster in an area enough to grow into a dangerous concentration. The faster the flow, the safer the water.
2) River water is often (not always) supplied by mountain snow and ice melt. This melt runoff tends to keep the river water at low of a temperature for microbes to live.
3) Lakes are fed by natural springs or rivers that provide a source of relatively clean water, preventing the water that’s there from getting too crapped up.
4) Barring destruction by human involvement, natural bodies of water almost always have their own ecosystems of plant, animal, and microbial life that control populations of bacteria or parasites that could be harmful in large concentrations.

Your pool doesn’t have a system of plants and animals to help maintain water quality, nor a source of consistenly fresh water to feed it, which is why you need chemicals to kill everything.

All that being said, as other commentors have pointed out, standing water should always be avoided, as they are often breeding grounds for parasites and very dangerous microbes.

Who says it doesn’t make you ill? Hepatitis A might not show any symptoms for a month and a half. When you get sick, you think you got it in the last few days, or a week, not 60 days ago or longer.

Also, when you get sick, it’s not always major symptoms. It could be one case of diarrhea, or a tummy ache for a day. Or you might be able to fight it off before you show any symptoms.