I'm rather new to this as well, but one of the things I've been doing lately has been adding two categories I've found on most Illness pages: Illness and Illnesses. I've found two different ways to do this.
The first is easiest. Go to the bottom of the page to where it says Categories, type in the category name, press enter, then save.
The other option is to use the Classic Editor. On the right hand side, under the Publish button is the Category section. Much like before, add the Category, press enter, then Publish.
The categories are for searching and listing ease. I think people would type illnesses to find them, but if we were going to list them anywhere we would want it to auto fill and Illness would be what we use.
Please do use the ones on the example pages and I will carefully go through them and clean up as things become more complete. ^^ Plus I actually just talked about this a bit in the editing outline/admin post, so hopefully as we formalise them and list important ones, it will become easier. ^^