[Sackett]

I think the key point is to realize that illness in fiction serves a narrative purpose, and the diseases chosen must fit those narrative purposes, and that the most common disease that fits that purpose is likely to be chosen.

Narrative Needs:

1: A temporary illness that weakens a character providing an opportunity for another character to nurse them. Almost always this will be: the flu, a cold, or a fever. Since the differences are not that big in terms of symptoms, they often won't even clearly categorize it and just show the symptoms.

2: A terminal illness. Results in death, but leaves the character alive for a while so that their death can be anticipated, providing angst. Cancer is the number one pick for this. Why? Everyone knows what it is, it can effect anyone, and it can be a killer even in societies with highly advanced medicine. They could pick something more specific but why bother?

3: A non-terminal wasting illness. This will make a character weak and dependent on others. Most often seen in an "Ill Girl" character. This doesn't have a "standard" like the other two, but also is often never specified (usually with the justification that the ill character doesn't want people to know). Sometimes handwaved as "anemia," "low blood sugar," or "heart problems."

Note that Japan's health care is pretty much as high quality as European or American medicine. Tuberculosis, to cite the specific illness you mentioned, is pretty much a controllable illness in these societies. Same goes for leprosy. They may not even be diagnosed because the high levels of nutrition available mean the bodies natural defenses can prevent serious symptoms from appearing. Those types of illnesses are more likely to show up in a period piece, thus demonstrating how medicine has not yet advanced.