Faolyn
(she/her)
Cure disease was lumped under lesser restoration. It's a 2nd-level spell, which rather suggests that in order to create a "realistic" world in 5e, where disease is rampant, either spellcasting clerics of 3rd level and above are vanishingly rare; disease, or at least deadly and/or epidemic diseases, are very rare to nonexistent; or clerics in general are seriously unwilling to heal without massive payments up front.The inclusion of parasites and diseases was, I think, largely to make rooting around in fetid areas more hazardous and-or force more drain on spell resources. One could argue this is yet another, though minor, "loss condition" present in earlier editions that has since been stripped out.
I'm not sure if 5e even has Cure Disease as a spell any more, but IMO it should; if nothing else it's probably the single most important spell to a stay-at-home Cleric trying to help those under his care, and it often has field use for adventurers as well.
(This is why I always say that you can't just include things and say "it's realistic!" because magic and nonhumans throws all that out the window. If your worldbuilding actively says that clerics are super-rare, then NPCs should be treating that PC cleric like an avatar--and then you have to hope that the players are OK with that.)
Anyway, the real world has twenty bazillion things in it, and it's impossible to produce a simulation for them all, at least not in a book format. So you have to pick and choose what's important for a very sim game to have and what level of detail is necessary. The disease section in the DMG is about two pages long and goes into rather disturbing detail involving each major organ group.
So... these things would need to be simplified. And how much can it be simplified before it stops being sim and starts being either nothing more than a flavorless mechanic (such as 5e's poisoned mechanic--disad on attack and ability rolls) or goes the other way and tells the GM and players how to model and flavor it, which is narrative enough to cause some people to break out into hives?