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
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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