We are not very good at forecasting outcome. Firstly there is the range of severity within a particular disease. Then there is the variation in human response to disease, both physical and psychological. The will to fight the disease can lead to "miracles"of survival. Finally there are the errors of diagnosis, coupled with the pathetic inadequacy of our knowledge. The world is full of eighty-year-old aunties who were told that they had "weak chests" and that they would not live a normal lifespan. Modern equivalents of weak chests abound. The diagnoses are more precise, but the acuracy of prognosis is about the same. In the individual patient who conronts you [or as you yourself, as patient, confront your disease! -- ZB], in the face of all these likelihoods and possibilities, optimism seems to be the most rational approach. it is certainty the most fruitful and least harmful.
I edited the quote to make it relevant to today's patient-centered culture, certainly a salutary change from thirty years ago when Mendel wrote his book.