One fear that many people have about growing older is a gradual decline of physical and cognitive function, so slow that it isn't quickly recognizable; perhaps you are just becoming forgetful, or delays in reaction time are simply a consequence of not being so young anymore. We see these changes occur in our grandparents and parents, hoping we might somehow be spared. We may; then again, we may not. Many neurodegenerative diseases are, at least in part, heritable. But some can also appear at random, subjecting us to the vagaries of chance. That's the uncertainty, the fear of the decline.