For centuries, prognosis was the principal thing doctors could offer to patients. They couldn't heal much, but they could say what was likely to happen. Their art was knowing the natural history of disease and imparting that knowledge in an authoritative way.
Today, medicine is awash in treatments that work, but the expectation that doctors can foretell the future has changed little. Every presidential election season, the candidates are asked to bring forth their doctors and medical records to, in effect, attest to their good health for the next four years.
That's happening in the current campaign, especially this week as Hillary Clinton, the Democratic nominee, takes a break to recover from what is presumably a case of community-acquired pneumonia.
Such infections can be caused by a long list of viruses or bacteria; it's often hard to identify the pathogen even if you look carefully. Diabetes, emphysema, cirrhosis and other chronic illnesses increase a person's chance of developing pneumonia, but in one-fifth of cases there's no underlying problem. Age, however, is an indisputable risk.