For those who are fans of Madeleine L'Engle's book, A Wind in the Door, I think I found something pertinent to the story.

In this story, Meg Murray's youngest brother, Charles Wallace, is sick with mitochondritis which is causing him respiratory failure. In the book, their mother is trying to prove that smaller organelles, called farandolae, exist and live in symbiosis with mitochondria, the way mitochondria are symbiotic with eukaryotic (nucleated) cells. In the story it is the farandolae that have something wrong with them and are endangering Charles Wallace's life.

Many a L'Engle fan has googled "farandolae" and come up empty, which goes to show how strong a fiction L'Engle weaved into her story. Instead, I took a different tack and looked up mitochondritis.

Mitochondritis was also called mitochondrosis in one journal article title I found: Myocardial mitochondrosis or mitochondritis: an electron microscopic study., and it is indeed a disease which destroys the respiratory mechanisms of the mitochondria. When this assembly-line-like set of metabolic processes (including such parts as the Electron Transport System, or ETS) stops functioning, byproducts and intermediate products form free radicals that can destroy tissue. One insidious effect of this accumulation is that these free radicals shut down other parts of the process, killing the system off even quicker.

The Cleveland Clinic for Consumer health information has this explanation for Mitochondrial Disease.
There's no cure, but diet and supplements (like Co-enzyme Q--essential for ETS) are used to manage it and delay the disease.

My guess is that Madeleine L'Engle had a dear one or knew of someone with this terrible disease and saw the heartache involved.

Ms. L'Engle's spiritual devotion shows in all of her books, and in this one she brings to the fore a most treasured aspect of her spiritual life: singing. In this story she gives form to things unseen, and she avows her belief in the connectedness of the universe through the music of the spheres.