Adding "an" or not, when describing different medical conditions - are there rules?

First off, I’ll admit that attempting to codify the use of "a" & "the" could easily drive a person insane. I hope you’ll forgive me for asking such a question.
However, I’ve got a specific situation for which I hope some solid rules can be found.
I have an ESL student who’s studying to be a Japanese/English medical interpreter.

I can’t seem to figure out a set of rules for when to add "a/an" to a medical condition, and when not to.

At first I thought I had it figured out, as follows:

If it’s a single occurrence of the condition and/or if it’s in a specific part of the body, then use a/an:

• A knee ache
• An aneurysm
• A tumor

However, if it’s systemic or can’t be pinned down to a fairly specific part of the body, then don’t use an article:

• Cancer
• Diabetes
• Heartburn

But this system breaks down when I consider that we say "chest pain", although it’s a single condition that’s specific to an area, while at the same time saying "tonsillitis" which is singular and in a very specific area.

I think (or hope) I’m missing a useful delimiter, here. Can anyone think of one?? ?