Donald Trump and Hillary Clinton. (Left: Jabin Botsford/The Washington Post; right: Melina Mara/The Washington Post)

David Brown is a physician and former science reporter for The Post.

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.

Video of Clinton’s departure seemed to show her buckling and stumbling as she got into her van. (Thomas Johnson/The Washington Post)

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.

So does Clinton’s illness mask, or foretell, a more serious health problem?

It could. But it probably doesn’t. More likely it’s the consequence of a transient dip in immunity — lowered resistance — from the strain of nonstop campaigning by a 68-year-old.

Nevertheless, an array of non-physicians (and at least one physician, TV doctor Drew Pinsky) has speculated that Clinton is hiding something big — seizures, Parkinson’s disease, a clotting disorder, general debility. At the same time, Donald Trump’s doctor has issued a statement that pictures the Republican candidate as an aging Charles Atlas, with “extraordinary” strength and stamina, certain to be “the healthiest individual ever elected to the presidency.”

Calls are growing for an independent panel of physicians to evaluate the health of candidates, as well as the suggestion that unedited medical records be available for public review. Before either option gains more traction, however, it’s worth asking where they might lead us.

In the standard visit to the doctor, most of the useful information comes from the “history” — the conversation with the patient — and not the physical exam. Would the candidates tell the truth? It’s hard enough for patients to be candid in utter privacy. How would they do in front of a panel of strangers who would soon report to the whole country?

Further, physical exams — in the absence of symptoms reported by the patient — actually reveal little. So little, in fact, that many doctors believe the “annual physical” should be abandoned. But what would happen if the panel found something?

The Fix's Aaron Blake explains the incident during which Democratic presidential nominee Hillary Clinton fell ill on Sept. 11, and why her health is likely to remain a subject of discussion. (Peter Stevenson/The Washington Post)

For example, a doctor occasionally hears a rhythmic “shhhh” when the stethoscope is pressed to the neck. Called a “bruit,” the sound suggests that the artery carrying blood to the brain is partially blocked, which increases a person’s risk for stroke. Whether to consider opening the blockage with surgery or a stent depends on how severe it is. That, in turn, requires more testing. Should the panel call for a work-up? Should the panel require electrocardiograms (EKGs) or stress tests, both commonly overordered in healthy adults? Would the panel make psychiatric diagnoses, which can’t be confirmed by lab or imaging tests?

Alternatively, perhaps the panel of doctors wouldn’t examine the candidates. Maybe they’d just review the medical records and tell us whether there was something to worry about. Would we believe them? What if there were dissenters? Could there be a minority report?

The less complicated route is to demand that candidates release their medical records in raw form, like tax returns. That would certainly reveal a lot. But what would happen if a candidate refused? Would public shaming be the only sanction? Would redactions be permitted? (How many aging men want the world to know they use Viagra?)

In the end, the question is: What’s enough information, but not too much? As with most things in medicine, the decision requires judgment as well as evidence.

It seems to me candidates should be expected to reveal their medical “problem list” — active and inactive diagnoses and conditions (“childhood asthma,” “Type 2 diabetes,” “non-cardiac chest pain”). This list used to be on the first page inside the paper chart; it’s still a key way physicians organize their thinking.

They should reveal what drugs they take on a regular basis — things such as Clinton’s anticoagulant and Trump’s cholesterol-lowering statin. As-needed meds (painkillers, Viagra) could be withheld. They should report weight, blood pressure, heart rate and a few key blood tests (measurements of kidney function, cholesterol, etc.). A case could be made for revealing recent diagnostic procedures and a summary of the results.

Clinton has come closer to this than Trump. Nevertheless, as someone who has had more demonstrable health problems (a fall, a concussion, a blood clot in the brain and now pneumonia) than her opponent, Clinton needs to tell us more than the bare minimum.

How far into the past this snooping should go is a matter for debate. More than one year, no more than five, would seem the right window.

None of this information, however, comes close to what we already have attesting to a candidate’s physical fitness.

It’s hard to hide serious illness. If either Clinton or Trump had cancer, a brain tumor, congestive heart failure, epilepsy, an untreated mood disorder or a half-dozen other things that could get in the way of a president’s performance, we’d know it by now.

They’ve been crisscrossing time zones, sleeping irregularly, eating things handed to them, not exercising enough and forswearing privacy for two years. They’re still standing (except for Clinton this week) and making sense (more or less). A presidential campaign is a marathon only the fit can finish.