I am not sure why you chose to show me this, as I'm not a medical professional. Anyway:
An immune system isn't failproof. It's subject to many random factors or influences and while we can try to keep it up and running, the chances are high that it fails sooner or later.
After 91 years, I'm actually surprised you don't have a lot more health issues.
The text you quoted says that it's treatable, so I guess it could have been worse :)