Make no mistake: there are levels of strength in one's immune response. People on chemo, people with diabetes (waves hand), people suffering nutritional/vitamin/mineral deficiencies, older people, babies and children, and the ever-popular category, people who are sleep-deprived, and many others all have a reduced immune response that can make them more susceptible to bacterial/viral infections in the first place and more likely to suffer serious complications if they do. And to an extent, raising your ability to fight off an infection is generally a good thing.
But saying that the immune system needs to get "stronger" or that a "natural" or "healthy" immune system is all you need is a very popular non-vax/anti-vax attitude, but it's one that can and WILL kill you. Here's why:
What kills people in strong epidemics and pandemics is not that the old and the weak and the immunocompromised are culled from the herd while the young, strong people with strapping immune systems mock us as we croak; no, it's the young, strong people with strapping immune systems die horribly simply because they DO have strapping immune systems.
(And ha ha ha ha ha ha HA! That'll learn 'em, we say!)
What actually happens when you've got an immune system that's capable of biting through 1/4" plywood is that a flu virus comes in that the system doesn't recognize--such as a pandemic version of H1N1, which actually is a real possibility for pandemic versions, along with H5N1 and maybe H3N2--that doesn't trigger the normal antibody responses and the body says "Holy shit, I have NO idea what this is! Let's mobilize EVERYTHING!" And because your body has an immune system that can eat nails and crap corkscrews, it does just that.
And alllllllllllllllllll the defenses are mustered....
...and a really unpleasant condition happens called a "cytokine storm." That's what tends to kill you in the nasty, pandemic varieties of influenza: it's not the flu itself; it's the bodily equivalent of scorched earth warfare. Your body is going to fight off whatever's happening to the last cell.
There are a lot of really unpleasant things that happen in a cytokine storm:
- multiple organ failures
- fevers that go off the end of the thermometer (literally; 106, 107 and *poof* you're dead)
- brain damage (usually from the fever)
- massive pneumonia caused by your lungs filling up
- everything choking, gasping, and shutting down
- death, death, death, and not at all pleasant--takes 24-36 hours of this: massive pain, the feeling of slowly drowning in your own fluids (which is literally what's happening), kidneys giving up and your urine backing up and you get toxic... It's a really bad way to go. Downright nasty, in fact. There are lots better ways to go. Certainly lots better ways to go that are years and years later, definitely.
(Exercise for the reader: look up "cytokine storm" and keep an eye out for what it says about the Spanish Flu pandemic. They happen with other really strong immunological responses, including SARS, hantavirus, and other things, certain types of transplant response, and other things where the body is being called on to fight off an extraordinary "invasion.")
So, yes, all of us older folks, particularly those of us who are also immunocompromised, will not fare particularly well, and some of us will indeed buy the farm, too, but it's the people who've been "training" their immune systems to be strong--on the "What doesn't kill me makes me stronger" basis, apparently--who are going to die from massive organ failure, 106-degree fevers, and having their lungs fill up with fluid over a day or two. (And when it's all over, if this didn't kill 'em, it certainly left them a lot weaker. They'll have months of recovery, large hospital bills like as not, and very possibly a seriously damaged body with many new medical conditions they didn't have before. Even if they recover quickly with no lasting effects, being sick unto death for 3 weeks is hardly my idea of a good time.)
So, for those people who want to train their immune systems: yes, eat right, exercise some, and get enough sleep and your bodies will do what they have evolved to do. For specific diseases, you can build up some resistance by exposing yourself to as many disease variants as possible, particularly every flu you can find each year... which won't help you a lot because the flu virus is amazingly good at evolving and changing to fool the body's immune responses. Or, as a better idea, you could get a fuckin' flu shot and not risk being sick for a couple weeks and even possibly dying during flu season. I mean, it's up to you how you wanna spend your time, but I really might suggest that the latter course would be a lot more fun.