Someone has an illness which is inevitably going to kill them. Why?

Imagine following situation: A person who thinks of themself as a good person, meaning they are mostly nice to people and act and think almost always morally. Said person is diagnosed with an illness that has no cure and is inevitably going to kill them in, say, a year. How would a Buddhist explain this? I already know about the four noble truths as well as the noble eightfold path. Also I know that one could explain it with the concept of rebirth. But I'd like a more in-depth explanation and I'm interested if there are other ways of explaining it or if I am wrong with the information I already have.