How Did The First Hokage Died?

The Battle of Uchiha Madara+Kyuubi VS Senju Hashirama

The First Hokage, Hashirama Senju, is told to be the most powerful ninja of his time. Not even Uchiha Madara can beat him even with the help of the Kyuubi, The Nine-Tailed Fox Demon. Just how powerful he was?!
But even then, we know that he died young. What on ninja world could be the cause of death of so powerful a ninja? The same question applicable also for his younger brother, Tobirama Senju, The Second Hokage, who also died in young age. These are the biggest mystery in the whole Naruto world for me. Any opinion are welcome.

If you don't know what I'm talking about, than you're a wanker!

From various source.