eli5: What is the goal of bacteria or viruses that make humans sick? Why do they weaken their host?
In: Biology
That’s not their goal. The only thing they want to do (and “want” is the wrong word, since they do not have any kind of thought process that leads them to want anything) is reproduce. That’s it. It’s an extremely powerful feedback loop – if a certain characteristic leads to increased reproduction, that characteristic is retained, enhanced and passed on. If it leads to less reproduction, that characteristic eventually disappears.
Also, the bacteria and viruses don’t actually cause fever or excess mucus production or other symptoms – it’s the body’s reaction to the invaders that produces these symptoms. There are countless bacteria and even small insects that co-exist peacefully in the human body – they live in our guts, bloodstream, hair follicles, etc., and the body does not go haywire in response. These organisms (and we humans) have evolved to “live together” so to speak. These organisms tend to be beneficial (like digestive system bacteria) or at least neutral (eyebrow mites), meaning they don’t consume so many resources from us that they threaten our ability to live and reproduce.
Combine these concepts and we get a system where the body “over-reacts” to invaders that haven’t yet “learned” how to live in us without causing a ruckus.
And then we have certain creatures that produce toxins as part of their existence (like botulism bacteria) but that’s an entirely separate discussion. Those creatures aren’t “trying” to live inside us – they’re perfectly “happy” elsewhere, but every now and then they get into our food supply and if conditions are right, produce a powerful toxin that greatly harms the body. But that’s not the same thing as a bacteria or virus that’s looking to use our bodies as a host.
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