My cat used to drink from the garden pond and never seemed to suffer any ill effects, and you often see dogs drinking from muddy puddles. So why do humans have to be so careful and only drink clean water?
鈥� The simple answer is: we don鈥檛. As a species, we survived many thousands of years drinking whatever water came to hand.
Many people throughout the world don鈥檛 have access to clean water, and have no problem most of the time. Of course, infections can spread through unclean water, so it is wise to be careful, but boiling the water or using a chemical treatment is usually sufficient to render it drinkable.
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Such precautions aren鈥檛 always necessary, and it is surprising what our immune systems can cope with. Once, during my military conscription, I found myself crossing a muddy creek, and low on water. I drank and filled my canteen from the stirred-up water we had just walked through.
We then walked some hundred metres upstream, where I saw the rotting carcass of a dead animal lying in the same creek. I had visions of being horribly sick, but fortunately, I suffered no illness.
Rivers into which industrial waste is discharged are different, but unpolluted water sources are generally safe as long as locals use good sanitation practices, and salt concentration isn鈥檛 high.
鈥淎s a species, we survived many thousands of years drinking whatever water came to hand鈥�
Hercules Gunter, Perth, Western Australia
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