I read a fairly long article that said he has been suffering from influenza for weeks.
Influenza kills tens of thousands in the US every year. It is not a trivial disease. Influenza is especially dangerous for the very young and very old. At age 57, Prince’s immune system may already have begun its age-related decline, making him more susceptible to serious disease.
Having said that, I will point out that it is way too early to speculate about Prince’s untimely death. Even if an autopsy has been done, the results have not been determined yet.
Living alone and trying to self-medicate and eschewing medical care didn't help.