When Hillary Clinton announced a diagnosis of pneumonia last week, soon after leaving a Sept. 11 memorial service, she elicited a predictably partisan response. Fans of Donald Trump speculated that she wouldn't survive the year, while her own supporters pointed out that hardworking people get sick all the time. Both presidential candidates have been pressured to release more information about their health. But this information may not be as useful as we think. Past assumptions about the health of presidents and candidates often have been shrouded in myth.

Myth No. 1

Franklin D. Roosevelt gave away Eastern Europe to the Soviets because he was sick.

As the Soviet Union took control of more and more of Eastern Europe after World War II, critics of Franklin D. Roosevelt argued that he - increasingly lethargic and confused because of illness - had been unfit to negotiate. The "sick man of Yalta," according to this theory, had been duped by his Soviet counterpart, Joseph Stalin. Lord Moran, Winston Churchill's physician who attended the 1945 Yalta conference that divided territory after the war, wrote that Roosevelt "intervened very little in the discussions, sitting with his mouth open. . . . I doubt, from what I have seen, whether he is fit for his job here."

The reality was probably much more complex. It is true that Roosevelt was suffering from severe hypertension and congestive heart failure, which the medications of the era could not effectively treat. And the trip to Yalta, located in the Crimea region of the Soviet Union, had been arduous. Roosevelt had suffered periods of extreme fatigue. But his personal physician Howard Bruenn, who was also at Yalta, observed that the president's mental faculties remained intact. "His memory for both recent and past events was good," Bruenn wrote in a 1970 article in the Annals of Internal Medicine. "His behavior toward his friends and intimates was unchanged and his speech unaltered."

Moreover, Yalta represented a complex series of negotiations on many postwar issues, not only the fate of Eastern Europe. Churchill, who was surely of sound mind, had participated in the negotiations and had trusted Stalin as well. And, as historian James MacGregor Burns has argued, the West had only so much leverage over the Soviets, who, after all, had suffered the greatest human sacrifices in defeating Hitler. Roosevelt, Burns wrote, was a realist who had "reached the limit of his bargaining power." His illness did not determine the fate of postwar Europe.

Myth No. 2

Sick presidents aren't good at the job.

Several books appeared in the 1980s and 1990s detailing the secret illnesses of American presidents and other leaders. With titles such as "The Impact of Illness on World Leaders" and "Ill-Advised: Presidential Health and Public Trust," the authors of these volumes showed that medical cover-ups were frequent in the White House. As Jerrold Post and Robert Robins wrote in their book on the topic, such leaders became both incompetent and manipulable. And it's true that Edith Wilson, wife of Woodrow Wilson, essentially ran the United States after her husband suffered several strokes while in office.

But there is no clear relationship between a president's performance and his well-health. For instance, Dwight D. Eisenhower had a series of medical issues while in office, including a heart attack, a bowel obstruction and a stroke. But historians have written approvingly of his presidency and his ability to achieve consensus. In a 2015 ranking of the presidents from a survey of political scientists, he came in seventh.

Perhaps one of the sickest presidents in the country's history was John F. Kennedy, who suffered not only from a failure of the adrenal glands, known as Addison's disease, but also from debilitating back pain. A physician, Janet Travell, treated Kennedy with narcotics, stimulants and various hormones. Most worrisome, Kennedy retained the services of a shady physician named Max Jacobson, who injected him with amphetamines. Yet while some of Kennedy's decisions, such as the ill-fated Bay of Pigs invasion, involved poor judgment, Kennedy biographer Robert Dallek found no definitive evidence that the illnesses or medications were the cause. And historians have praised many aspects of Kennedy's tenure, such as his handling of the Cuban Missile Crisis, and his efforts to improve poverty and race relations.

Myth No. 3

A clean bill of health during the campaign is a good predictor of health in office.

The revelations about past cover-ups led to calls for better disclosure from candidates and officeholders. In 2008, a group of prominent doctors proposed forming an independent panel of physicians who would examine all potential presidents and vice-presidents. Connie Mariano, who was the White House physician in the Bill Clinton and George W. Bush administrations, favored this strategy, comparing it to the screening of airline pilots or military personnel. The message, she told CNN, would be: "Okay, I'm good for four to eight years."

But being healthy at one point in time may have little relevance to what happens during a future presidential term. George H.W. Bush was healthy when he assumed office in 1989, but in 1991 he developed atrial fibrillation, a potentially dangerous heart rhythm. Doctors subsequently diagnosed Graves' disease, an excess of thyroid hormone that had caused the heart abnormality.

Perhaps the most telling example of false reassurance was the case of Ronald Reagan. Reagan, who was 69when he ran for the presidency in 1980, authorized the release of information from his doctors. They raised no concerns, describing him as in "excellent health." But many commentators, including Reagan's son Ron, now think that Reagan was showing signs of Alzheimer's disease by the time of his second term. Even during the campaign, Ron Reagan said, his father looked "tired and bewildered" during a debate against Walter Mondale.

Myth No. 4

The doctors of presidential patients tell the public the truth.

When Eisenhower suffered his heart attack in 1955, he decided that Americans deserved to know what had happened. As he recuperated, his doctors held news conferences to educate the public about heart disease and to detail the president's condition. This type of disclosure became routine over the years, as in the aftermath of the assassination attempt on Reagan in 1981. Reagan's doctors took this task seriously; one doctor stated that the hospital "recognized the need to provide accurate information to the news media."

In practice, however, presidential physicians have lied, often blatantly. Ross McIntyre, Roosevelt's first White House physician, continually characterized his patient's shortness of breath - from congestive heart failure - as a sinus problem. The most egregious lie, perhaps, was one of omission. When Paul Tsongas was running for president in 1992, his hematologist confirmed his patient's history of lymphoma but not that the disease had recently recurred and thus probably was incurable. Had Tsongas been elected, he would have died two days before completing his first term.

Myth No. 5

Presidents shouldn't, and don't, have some mental illnesses.

In July 1972, Thomas Eagleton, the vice-presidential nominee of George McGovern, was forced to withdraw his candidacy when reporters discovered that Eagleton had been hospitalized for depression three times, including receiving electroshock therapy. Common consensus held that he could not serve. McGovern, one newspaper editorial read, "does not need the added burden of a presidential running mate with a precarious health problem in the area of mental illness." In 1988, presidential candidate Michael Dukakis aggressively denied any history of depression when rumors surfaced. Dukakis, his physician told the news media, "has had no psychological symptoms, complaints or treatment."

A 2006 study of the first 37 presidents concluded that 18 of them had some type of psychiatric disorder during their lives, ranging from depression to anxiety to alcoholism. Ten exhibited symptoms while in office. Although the paper concluded that these conditions had negatively affected aspects of their presidencies, some of those affected were nevertheless extremely successful leaders. One of them was Abraham Lincoln, who is routinely at or near the top of "best presidents" lists, but had a major depressive disorder with psychotic features. Woodrow Wilson, who suffered from depression and anxiety in office, was listed at number 10 on the 2015 list. And Teddy Roosevelt, the study concluded, probably had bipolar disorder during his presidency, but his symptoms did not interfere with his effectiveness or performance.

Lerner, a professor in the division of medical ethics at New York University's Langone Medical Center, is the author of "The Breast Cancer Wars" and, most recently, "The Good Doctor."