OneFist
All people who die, stop breathing. Therefore one could conclude that failure to breathe is what kills all humans. Thinks about it. When was the last funeral you went to where the corpse was still breathing? All other causes of death are fake. It's a giant conspiracy of doctors, pharmacies, and health insurers. They don't want you to know that if you keep breathing, you would be immortal.
Day 2^9 +2^7 +33 | I'd definitly say Time is up there, after all no one can survives it's grinding power for long. Maybe hunger/starvation? This, like time, has always existed till now. And to add the previous, thirst/(idk the word)? Oblivious d3234232 and 1lln3232 are also up there. Maybe not a strong one like the plague, more so one that has followed us humans since forever and only is fatal to the weak.