These are interesting thought experiments. Yes, in
Post# of 148174
And in the real world, separate from theory, many, many immuno-compromised patients are full of bizarre organisms and hard-to-treat strains. They don't all just succumb to garden-variety germs, like wounded zebras succumb to their predators.
One of many examples: the nasty treatment-resistant TB, first appeared in southeast Asia, then spread elsewhere. Standard treatments don't work; maybe a partial response with a subsequent stew pot of residual smoldering infection. Plenty of time and opportunity for mutations. Gee, how did I get that, doctor? Well, we don't always know, sir/ma'am.