He had been reduced to the condition of an ancient invalid and it took him long, long minutes to crawl across his room — crawling over the ceiling was out of the question — but this deterioration in his condition was fully (in his opinion) made up for by the door to the living room being left open every evening.
There are no more uses of "deteriorate" in the book.
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The situation could quickly deteriorate.
Writing, at its best, is a lonely life. Organizations for writers palliate the writer’s loneliness, but I doubt if they improve his writing. He grows in public stature as he sheds his loneliness and often his work deteriorates.