My mother calls her arthritis “Arthur”, as it “Arthur is back today.” I always took it to be a euphemism–like when women talk about “Aunt Flo”–rather than the result of a liguistic proces like epenthesis.
As for arthur-itis, it’s not all about sloppy enunciation. Codgers have always had a kind of black humor about their ailments. “Going to bed with Arthur” and such are jokes we’ve all heard countless times, but we recite them anyway. It’s a small way of saying that we can laugh in the face of inescapable diseases, and we are not defeated.
Calling it “Arthur” may be a deliberate piece of euphemism. (I’m not even certain that “euphemism” is the correct word. I don’t think that people would do it because that consider “arthritis” to be a taboo word. They would do it to be cute and have a nickname for the disease.) Adding a vowel in the middle to make it sound like “Arthuritis” very probably isn’t a euphemism. Epenthesis is a very common sort of linguistic change, and it isn’t deliberate in general.
I don’t know if you’re talking UK English, but in US English, at least, going “to the school” and going “to school” have different meanings. “To the school” means to the building and usually implies you are not a student there. “To school” means you are going to a school as a student.
However you will get “a cold” or “an allergy”. This is just the difference between the definite and indefinite article. Because you only get the flu once a year and measles once in your lifetime, you are talking about something specific. But if you get a cold, the symptoms are common enough that you can’t really pinpoint which variant you have.
To extend this, most people say they have arthritis, not “the” arthritis, since this is something that you have as a condition, rather than as an event with a finite length.
In most cases, Brits and Americans use the same usage here; Americans just stopped using it for “hospital”, for some reason.
A criminal goes to prison. Qadgop the Mercotan goes to the prison.
A student goes to school. A parent in the PTA goes to the school.
A parishoner goes to church. The guy who’s hired to repair the organ goes to the church.
See the pattern here? So far, this is the same on both sides of the pond. Likewise, in British English,
A sick person goes to hospital. A nurse goes to the hospital.