Eli5 if disease’s want to live in a host’s body without being discovered, then why do they have such nasty symptoms that can be used to easily identify and destroy them?
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I think the fundamental problem with your question is that you assume intent – “want” – when no such thing is present.
The microorganisms that cause the kind of diseases you’re referring to are, essentially, “infinite monkeys” made manifest; The old saw being “if you have an infinite number of monkeys bashing randomly away on an infinite number of typewriters, eventually you will get the entire script to *MacBeth.”*
Microorganism-based diseases, viruses in particular, are little more than chaotic combinations of proteins that random chance has caused to have certain effects. The “Ideal” virus, for instance, would be one that, upon infecting a human, grants them immortality or as close to it as possible (so, agelessness), regresses their age to peak physical health if they are older (so, it “wants” a host of ~25 or so forever) *vigorously* fights off all other diseases whether viral, bacterial, or fungal, is not itself recognized *as* a disease by the host’s immune system, and promotes healing and wellbeing in all things (so, for example, it would have to cause a human to burn off excess fat in the event the host partakes of too much). The “Ideal” virus, thus, is in fact a *symbiote,* and if one should arise, it would clearly immediately outcompete *all other viruses* and infectious microorganisms in the “propagation of its own code” field.
And if that sounds fantastic to you, well, it sounds fantastic to me, too, but you’re *far more* likely to get, out of infinite monkey-operated typewriters, something that, if run through a few spellcheckers and word-checkers and maybe subject to some AI auto-correction, something that reads like an absolutely *godawful* *Sailor Moon* × *Firefly* × *My Little Pony* × World War II × Roman Empire fanfiction/alt-history set in Holmesian London, as written by a hyperactive nine-year-old let irresponsibly loose with a word processor that takes dictation.
That’s kind of what infectious diseases *are:* not the products of “want,” implying that they can identify a goal and adapt themselves to it. They’re organisms, and arguably not even organisms in the case of viruses (it might be better to consider viruses to be out-of-control organic nanomachine swarms), that act in the manner that their genetic structure makes them suitable to do, which is to linger around wherever they may be, and do their thing. Over countless iterations of this, when they reproduce, some of those reproductions will have mutations in them. Imagine if the “common cold” (which isn’t actually one disease, *at all,* but for the purposes of illustration we’ll assume that “the cold” is one virus) only transmitted through urine ingestion. It wouldn’t get very far! In fact, the first case of the cold might well have been the *last* case of the cold!
Except, some of those “The Common Cold virons” mutated in the original host in such a way that they could become airborne and infect another human or animal through getting those viral proteins in their mucus membranes. Suddenly transmission is *much* more viable; it’s *far* more likely that someone will sneeze in someone else’s face, than urinate upon it. Now “The Common Cold” is in *business,* and the strain that gets into the next person will reproduce a lot, and those reproduced copies with their “infection through airborne transmission” mutation will be *a lot* more likely to infect others.
But just imagine if, instead of sneezing, humans just… Didn’t. Did not sneeze, did not cough. Perhaps assume that, somehow, we evolved to *fart* in the circumstances when we now would sneeze.
Then that “infection through airborne contact with mucus membranes” mutation becomes a *lot* less useful and likely to result in successful propagation. Not *entirely* useless, but it’s a lot less likely that humans will fart in one another’s face than that they will sneeze in it – though still *somewhat* more common than urinating in it.
Evolution is not “survival of the fittest.” It is “survival of the adequate.” Just imagine, if you will, if that “Ideal” virus actually existed, that it mutated from the common cold, but its only transmission mechanism was sexual. Sounds good to me! But now imagine if the first human it actually infected was an old ***old*** school Catholic priest circa 670 AD. A True Believer type, who never in his life ever has sex. Whoops! Now that “ideal” virus is going *nowhere,* and will wind up extinct when its host inevitably dies of mishap or natural calamity (a wind-storm knocks a tree over and it falls on him), violence (Scandanavians going a-Viking come upon him in a lonely hermitage in 955 and put a spear through him), or other form of misadventure (he mistakenly believed himself a Saint empowered by God, or possibly even a return of Jesus, so he went to Ireland during the potato famine to help, only to starve and die because he was being kept alive by an organic nanoswarm, not the divine). Hell, that guy might not be dead *yet,* but if he’s not transmitting the agelessness virus, it’s not going anywhere, and if it can keep him alive, it probably has very good *self* repair mechanisms, so it’s not mutating much within him either.
So a virus like Covid-19 has nasty symptoms that can and do *kill* their victims because it is not made with any kind of “intent” or “desire” to live in a host’s body without being discovered. It gets into someone and it starts reproducing wildly, hijacking cells nonstop because that’s what its fucked-up, random, chaotic organic programming and resulting protein shape causes – and many of the symptoms we notice are the result of the body’s immune system attempting to fight it off anyway, and using a lot of your metabolic energy to do so.
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