Why does J. in Jerome's Three Men in a Boat consider himself an "interesting case" and a "class acquisition" from a medical perspective?

After going to the British Museum one day to read up on some "slight ailment" he suffers from, J. decides he has almost every illness in the medical books, from Bright's Disease to cholera. This leads him to believe a classroom of medical students would benefit from him greatly: since he has so many diseases, they wouldn't have to walk hospital wards looking for patients to study. They could simply study him in the classroom.

This is part of the comic hyperbole (exaggeration) that makes the novel funny: we all have a tendency to think we have the disease we're hearing about, and Jerome takes this to an extreme. However, it serves another function. George and his two friends, George and Harris, discuss J.'s situation and decide they all need a complete change and rest. This becomes the catalyst for the trip on the houseboat that is the subject of the book. 

The narrator, J., tells this story in Chapter I. He had gone to the British Museum to use a health encyclopedia, and he was compelled to look at the lists of symptoms for each and every disease in the book. As he moved from “ague” to “zymosis,” J. convinced himself that he had some variations of nearly every malady except one: “housemaid’s knee.” (Naturally he wouldn’t suffer from this ailment, since he would never be one to get down and scrub a floor.) But J. put a good spin on this realization. He could farm himself out to medical school students. They wouldn’t even have to go to a hospital to study multiple patients, because he had it all in one package. They could merely study him: a singular disease compendium. “All they need do would be to walk round me, and, after that, take their diploma,” he said. Whether he was truly a hypochondriac, or was using exaggeration for humorous effect, is a decision left to the readers.

Why does the narrator in "Three Men in a Boat" consider himself an interesting medical case?

This reference appears in the opening pages of “Three Men in a Boat.” The narrator recalls a time when he went to the British Museum to discover a treatment for hay fever. He searched through a medical book for the answer, and then began to turn pages at random to read up on other ailments. To his horror he found that he had the symptoms of almost every disease or malady that was listed. At first he thought this status would make him a perfect example for medical study, since he suffered from everything.

“What an acquisition I should be to a class! Students would have no need to ‘walk the hospitals,’ if they had me. I was a hospital in myself.”

Then he began to worry about himself and his real health. Savvy readers should realize that the narrator is prone to exaggeration and tends to react disproportionately to many situations. He is setting the tone for the rest of the action in the book.