Spoilers? Spoilers:
I want to revitalize this thread because of the recent issues including Madara and Hashirama's history. Namely, I disagree with all y'all. I think there was an oversight and the author didn't take into consideration how in the hell Hashirama would die of anything other than old age. Too bad he died in the war?
Let's look at some statistics: 1 Raikage=10,000 shinobi. 5 kage>1 Raikage. 1 madara=1 Bijuu> 5 kage. Hashirama> kyuubi+madara>several friggin bijuu.
Theory that the Madara battle tuckered the poor sucker out: bupkis, e'erbody gets super jacked using this mofo's cells let alone his chakra. He can heal up right quick. friggen' granddaughter nearly dies and heals up, why wouldn't he?
Theory that Hashirama got teh AIDS: refer to previous logic, goddamn mofo makes people live HELLA long. Hashirama cells= cure to AIDS.
Theory that Hashirama had a whoopsie during battle: statistically speaking, if a mofo can shy away from making a whoopsie against 10,000 shinobi, and then get severely outclassed by some other mofo; good chances are that the latter mofo gonna need hella shinobi to accidental all over his HP bar. I'm talkin' entire 2nd world war against him.
EDIT...I realize that some of my stats are off...Raikage didn't technically die at the hands of 5 kage...but can we agree that one of several hundred Naruto shadow clones is a lesser? Aside that though, most other errors would only make Hashirama look even more unstoppable. e.g. arguing that Madara's recent Susano'o was the real reason we can say he equals a bijuu would lead us to conclude that this was only because of Hashi's cells.