Doubt

 

Because it spreads throughout your life.
Because it consumes you, in the end; in the end, malignant versions of yourself have destroyed far too much to allow for sustained survival.
Because you become terminally ill with it. Upon analysis you find that the core is hollow. That the life is a shell.
The body is a vessel, and when the vessel is broken, there is nothing.
A big, awfully blank 

NOTHING.

The conversion of cells, the mutation, proving evolution on a cellular level.
The wirings of the brain much like that of the television (have you ever seen it when you turn the power off?), residual electricity surges and points in the center, a bright beacon of light through the tunnel.