Sepsis. He had a pretty bad infection (a couple of boils, actually) that he didn't get treatment for, because he thought they would just go away on their own. Except they got really REALLY bad. And boils are caused by staph infections, which can be super serious when they become blood poisoning (sepsis), which can cause all of your organ systems to shut down and you die.

Which is pretty much exactly what happened. What is so stupid was that a bunch of hefty antibiotics even a day or two earlier (before he was found in a coma in his house) would have taken care of the whole mess.