What makes it clear that the author of 'Disease Maker's Croup' is suffering from the disease himself?

The author of 'Disease Maker's Croup' begins the description of the disease very sensibly, describing the initial and secondary symptoms. However, as he begins to describe the way the disease affects speech and thought in the tertiary stage, his description suddenly seems to make a lot less sense. As he describes the condition worsening, his writing becomes even more nonsensical, rambling on about different afflictions and body parts. He also begins to repeat himself quite a lot, something else that he identifies as a symptom of the disease.