both the public consciousness¶

A pernicious mythology that has flourished largely unchallenged in both the public consciousness and the medical profession is the concept of the immune system as being "strengthened" by infections. The idea goes that your immune system starts off inexperienced and vulnerable, but through exposure to various pathogens, it becomes generally better at fighting pathogens. It's envisioned as a military being hardened through combat experience. This isn't entirely wrong: your immune system has an entire branch -- out of several branches -- dedicated to "adaptive immunity." It stores fragments of pathogens (or rather, what it thinks are pathogens) and uses them to create targeted responses specific to that antigen. However, this is NOT something that makes your immune system "stronger." It's more like differentiation -- the process of cells and tissues changing to be able to perform a highly specialized job. They forgo the ability to adapt in order to get better suited to one task. Think of how every* cell in your body contains the same* DNA, but eye cells and liver cells perform completely different jobs. As you develop, cells in different parts of your body are exposed to different biochemical signals and develop into different cell types. Throughout the process of differentiation, cells pass various thresholds after which they can't go backwards and develop differently. Eye cells can never* become any other cell type once they've fully matured. And that's good! Otherwise, your eyes would get messed up! A good analogy is the way we specialize into fields of expertise. We start off with no education or experience, then we might go to college for biology, then we might go to grad school and learn developmental biology, then a postdoc in the differentiation of eye cells, etc. The more specialized you get, the better you are at accomplishing your niche thing. But that comes at the cost of your potential to specialize in other things. A 50-year-old tenured professor is no longer that fresh-faced high school graduate exploring their options. Of course, as people, we could always go back and start training in a new field, adding to our experience. It's never too late to learn new things. But it's certainly much harder for that professor to start a new career from scratch and gain that same level of expertise again. Looking back at the immune system, the process is MUCH more grueling than human specialization. Colleges don't typically slaughter the bottom 95% of the class. Immune cells are expected to become HIGHLY specialized, very quickly. Especially during an acute infection. Specialization of immune cells is just one aspect of adaptive immunity among many, many others. And even adaptive immunity itself is just one branch of the astonishingly complex immune system, most of which has nothing to do with "training" on specific pathogens. Some components don't even have a clue about the difference between viruses and bacteria. Some can be dumb enough to think a peanut protein or a microgram of gluten is a dangerous toxin that needs to be expunged NOW, RIGHT NOW, EVEN IF IT KILLS US, WE WILL TAKE IT DOWN WITH US. Your immune system is also not a limitless supply. Even though your body is constantly breaking down and refreshing old parts and generating new cells and molecules, this process degrades over time. This is why older people have weaker immune systems, not stronger. If your immune system was an elite fighting force, constantly getting better with every victory, our elders would have the strongest immune systems of all of us. Instead, a lifetime of fighting has caused their immune systems to begin to lose steam. Not only are their physical reserves running low due to diminished capacity to resupply, what they have has been "trained" on enemies that they aren't facing. We are constantly bombarded with new pathogens. Some are entirely novel (like when COVID first emerged). But most are just new versions of things you've kinda seen before. In the 80 years since your first flu, the strain you "trained" on has evolved through more generations than those separating us from gorillas. Could a retired marine squad fight a bunch of armed hypergorillas? During especially nasty acute infections, it's all hands on deck. The immune system goes into overdrive trying to figure out how to deal with this existential threat, nuance and restraint be damned. Who knows if you'll live to see another infection, THIS needs to be dealt with! Your immune system, quite simply, is never the same again after an ordeal like that. It is primed to fight a particular enemy, resources have been expended willy-nilly, collateral damage has been wrought, and it can be hard to even transition out of that "total war" mode. Especially for pathogens like SARS-CoV-2, which can linger for a loooong time. You might feel better, but your immune system is still hard at work. And it's still "training."

Sometimes, it makes mistakes during this protracted response. It misidentifies enemies. Autoimmunity is still poorly understood, but one aspect of it is essentially that various parts of your immune system, already primed to fight a seemingly-omnipresent enemy, go haywire trying to identify enemies, and lock onto bits of your own body as valid targets. They can specialize in targeting harmless, normal human cells, and never be satisfied that the infection is gone: they keep triggering waves of attack, as long as the "enemy" refuses to go away -- because it's YOU they're fighting! In addition to damage being done to those particular tissues being targeted, this also drains your resources even more. You feel sick and weak, even when you aren't infected. When you do get infected with something new, your immune system's attention is divided. This is all just a tiny, oversimplified snapshot of some of the dynamics at play in the immune system. It's unbelievably complex, arguably the most complex system in your entire body, the most highly variable between individuals and populations, a key focus of our evolution. The immune system is not something to be taken for granted. It's not some limitless resource to be drawn upon constantly. It's not even close to infallible, and makes way more mistakes than you imagine. It's certainly not a muscle to be exercised and made stronger.

Protect it. *These are generalizations. The human body is extraordinarily complex and subject to all sorts of weird stuff going on. Somatic mutations, cancer, ectopic tissue formation, and so on, make this all impossible to present as being 100% true 100% of the time.