Someone who is hallucinating will see things, whole scenes and events which are not actually there, and experience them as real.How can this be? Because you see what you expect, unless it is corrected by your perceptions. Which is where things can go wrong.
Disease can be seen as the antechamber of death.
Are there reliable tests for predicting disease? The short answer is: not for Alzheimer’s or other common diseases. Tests do exist for some specific diseases, most with known familial predispositions, such as Huntington’s disease, cystic fibrosis, Duchenne’s dystrophy and many more.