Something about some things that can't be fixed.
With a daughter in Bio and my natural curiosity, I read up a bit on Just How The Does The Immune System Work, Anyway?
Any real doctor/biologist who reads the below and wants to rag on it, be my guest. I'm a tyro.
Directly beneath one's breastbone lives a little organ where T-Cells, of both the Killer and Helper types are born. An infant gets a fresh, really active one of these. New T-Cells get born all the time and filter their way down through this organ. Apparently, said organ had samples of different kinds of tissues from All Over Ye Body. If a T-Cell doesn't react properly to the tissues, like it's supposed to, the organ kills off the malfunctioning newly-born T-Cell. T-Cells that survive this cull go through more tests; those that fail, get killed, the rest keep on going.
At the end of this process, by some means that I can't remember, the T-Cell either become Helper T-Cells or Killer T-Cells and migrate into the body; my understanding is that they hang out in the Lymph system, or at least the Helper ones do.
The Helper T-Cells have both genetic memory of previous infections and other data. They hang out in the Lymph. If some immune cell kills off some infected cell, it places proteins from the now dead cell and proceeds to the lymph system, then bangs against various Helper T-Cells until one or more of the T-Cells recognizes one or more of those busted dead proteins and "thinks" it can do something about it, that something being making antibodies against the infectious agent that it's being exposed to. A flood of antibodies then ensues, killing off even more of the infectious agent, more bits and pieces get dragged back, and the human body kicks into high gear or something.
All well and good. However, remember that organ under the breastbone where all this got started? As one gets older, it wears out, becomes fatty tissue, and doesn't work so good no more. And
that is one of the main reason that older people get sicker, easier, when exposed to a novel virus.
(And, to some extent, once the body and all those helper T-Cells have been exposed to an infectious agent, the helper T-Cells tend to react to "I've seen
this before" agents quickly. Which helps people who've been alive for a while, so being old isn't
all bad.)