What do viruses/bacteria exactly do inside of our bodies that makes them dangerous and causes us to get sick?
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The short answer is they grow and replicate. However, in order to do this they attack and destroy the cells in our bodies. Viruses, for example, contain none of the machinery of their own to stay alive. In fact, ad you may know, they are rather famously considered to not even be alive. In order for them to replicate, they need to infect a host cell, hijack the cellular machinery, and start using it for its own purpose. The cell becomes entirely devoted to making more and more virions until eventually the cell is so full with viruses that it rips open, flood g the body with thousands and thousands of new virions to infect new cells.
So diseases have become specialists, and only infect certain kind of cells. Chicken pox and rabies, for example, both hide in nerve cells to stay hidden from the immune system and lurk unseen.
They both act a little differently in our body. The main problem is they use cell material in our body that we would otherwise use to stay healthy. By taking that material, internal processes are interrupted and can no longer keep all our cells healthy.
You ever see a wasp lay eggs inside of a spider?
It’s kind of like that. Viruses infect human cells, replicate inside of them, and then tear the cell apart. Now you have one less cell in your body and thousands of new virions (an individual virus isn’t a cell itself, it’s a virion). They infect thousands of cells and the process repeats. I’m sure you can see how this can get really serious, really fast.
An infection is an overgrowth of a pathogen, too much of a bad thing. Your body plays hosts to lots of beautiful good bugs that cause no problems at all! Symptoms can vary depending on the bug and the degree of overgrowth. Some bacteria straight up produce toxins that can make you very sick by targeting a necessary protein. Sometimes the presence of bacteria activates a nonspecific immune response (inflammation) and that creates the runny nose, headache, fevers… but those are actually just responses from the body trying to control the overgrowth, not the bug itself!
In the case of Covid, the body has such a strong response to the virus that it creates a cycle of inflammation that clogs up the lungs. So in that case, it’s not the virus but the body that makes you really sick.
Just a little additional info:
When you “feel sick” most of these symptoms are not caused directly by the infection, but by your own immune system attempting to kill/flush out the infection.
Fever to kill the infection
Runny nose (increased secretion) to flush out the system
Cough/sneeze. To expel built up secretions and infectious organisms.
In fact allergies are just your immune system reacting to things that aren’t pathogens.
ELI5 answer:
You own a house. A lovely house. One day your kid shows you with a mate. You allow them inside. Then they ask to stay a bit, you see no issue so you say yes. Then they kill your kid and take over him like some sort of frankenstein pinnochio.
Then you start to see more of him. Lots more. And it gets worse until one day he strangles you in your sleep and claims your home as his. And he starts making more kids like a redneck with a car full of moonshine and meth.
Then, he sends thise kids to the neighbours house to do the same thing. Maybe they call the cops, maybe they don’t. But even if they did his kids just shoot them and take their weapons and cars. This goes on for months until the president (your brain) nukes them.
They die in the fire but not before doing massive damage to everyone involved.
So, as have already been said, viruses hijack your cells and replicate. The symptoms are your body trying to get rid of it, fever to make the host environment no longer tolerable, mucus and diarrhea to flush it out.
Bacteria are another thing altogether. Bacteria want to eat and grow just like other organisms. Bacteria are already in you and on you. Forming a mostly healthy symbiosis, like the ones in your intestines that help you digest food.
For the most part, bacteria are harmless. However, sometimes there is an imbalance in your body that allows them to grow out of control. Other times you are exposed to bacteria that your body isn’t adapted to. In many cases what kills you are the byproducts of the bacteria, not only are they consuming your cells but they are creating “waste” that are poisoning you.
Many have explained what viruses do, but failed to address the issues with bacteria.
Bacteria do not need live human cells to survive. But many still release toxins that kill our cells. Others don’t but most release products that activate our immune response and inflammation. If that spirals out of control you get sepsis, if not – a fever.
On the other hand, some bacteria are good at avoiding our immune response (tuberculosis, syphilis, Lyme disease). They grow and multiply in some tissues until slow and constant inflammation and some of their toxinss cause damage.
One mechanism that has not been mentioned yet is that both bacteria and viruses can damage your DNA, inflame your tissue or suspress your immune system and cause cancer.
Look what happens when you have no immune system (when you die). You literally rot away as infections, bacteria, fungus go wild on all the organic material that is no longer actively being defended.
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