Dysentery. Look how many times the great warlords of Europe have been stopped by the simple fact that they don’t wash their hands and the history has been changed. If LoTR was anything like the middle ages (which it doesn’t try to be, it models itself more according to the great sagas) Aragorn would just die a horrible death of shatting blood in insane pain.
I recentrly read Graves’ Goodbye to All That and his description of the trenches is exactly like Mordor. But there’s no elves and hobbits, it’s just orks fighting orks. Reading it as an adult at the whole book is extremely coloured by Tolien’s experiences in First World War (he was in Somme, the poor bastard) to the extent you might call it the best book about The Great War. Of course none of this I understood when I read it at 12-years-old. It was all elves and dwarves and balrogs and good vs evil. But I grew up.
But you're right, of course. (Also, Aragorn might have died of tetanus, or an infected, minor wound; though there is an implication, in LOTR, that within the world of the story, the Elves have a brought to Middle Earth a sophisticated knowledge of medicine, along with everything else. Aragorn became a reknowned 'healer', etc, from staying with them. So you're right; LOTR inhabits a different world altogether from our Middle Ages).
We can't remotely imagine the shock and horror it must have been for men like Robert Graves and Tolkien, to know only picturesque townships and bucolic landscapes, and THEN abruptly be thrust into the hellscape of the Western front. (The recent movie, All Quiet on the Western Front is extraordinarily good at showing how hellish it was. Brilliant, in that respect, though I warn you: it is not a good time). But again, these considerations, regarding history and the culture of the past, are utterly beyond the scope and capacities of the neo-feudalists. They have no conception of the world they're creating, nor how ill-suited they are to inhabiting it.