You see all kinds of animals drinking from puddles, ponds, etc and they are fine, whereas us humans can’t do it without getting sick.
In: Biology
Also a lot of animals have shorter GI tracts, so bacterial growth time is shorter. Some also have more acidic stomachs that are better at killing parasites and bacteria.
Overall it’s a lot of survivor bias though. We look at mature animals and think, “how do they survive”, but don’t see the ones who died.
The easy answer to this and all similar questions is animals babies/children die at rates prior to adulthood that we would consider unacceptable for humans in the developed world. Animals also rarely live to what we’d consider old age. Consider the lifespan of most animals in captivity, versus out of captivity where they live much longer.
Drinking clean water is one way we prevent babies/children from dying early and keep older, and somewhat unhealthy people from dying as well. Not to mention, not carrying a bunch of disease and parasites helps you be healthier overall.
In part, wild animals have better, more practiced immune systems — because they have to!
Most humans in developed societies have immune systems that are not practiced against many pathogens because developed societies put a lot of effort into keeping people from coming into contact with those pathogens.
It’s not necessarily that our immune systems are “weaker”, it’s just that our immune systems have no experience with those things and thus our defenses are weak against those pathogens. Your immune system needs ***practice*** in order to deal with pathogens. But, the math works out that avoiding the pathogens still comes out as statistically better than not avoiding them.
Humans today that do not live in developed societies drink water and eat food that would cause us in developed societies to get sick. They are, of course, more likely to die (especially at a young age) and have much higher incidence of illness. But, they can go drink out of their local river and 99.99% of the time be just fine because their immune systems are well practiced against the pathogens there.
This is also why children tend to get sick much more often than adults (until old age at least). Over your lifetime your immune system gets more and more practiced against various recurring pathogens and you won’t get sick as often.
Finally: this is why vaccines work! Some people think vaccines are like drugs or antibiotics and directly fight viruses. They do not. All a vaccine does is give your immune system *practice* in a way that is not likely to actually result in harm! It’s like army dudes that practice with paint ball guns. They can get real world practice without getting shot with a real bullet.
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