As President Trump remains hospitalized at Walter Reed National Military Medical Center after testing positive for the coronavirus, one doctor is at the center of his treatment: Sean P. Conley, the White House physician.

Stepping out of the hospital with a team of doctors behind him on Saturday, Dr. Conley gave an optimistic update on Mr. Trump’s condition at a news conference. He said the president was “doing very well” and in “exceptionally good spirits” after spending Friday night at the hospital.

The news conference put a national spotlight on Dr. Conley, who offered a distinctly different outlook from what Mark Meadows, the White House chief of staff, told reporters later.

Here’s what we know about Dr. Conley.

Dr. Conley took on the role of White House physician in 2018 after Dr. Ronny L. Jackson was nominated to be secretary of the Department of Veterans Affairs. Dr. Jackson had to withdraw his name from consideration for that post amid accusations of inappropriate workplace behavior and was subsequently promoted by Mr. Trump to the position of assistant to the president and chief White House medical adviser. He is now running for a House seat in Texas.

In March 2018, Dr. Conley was named acting White House physician, and he was officially appointed to the position by Mr. Trump in May 2018.

Dr. Conley graduated from the Philadelphia College of Osteopathic Medicine in 2006, according to records from the Virginia Board of Medicine.

Doctors of osteopathic medicine, or D.O.s, are fully licensed physicians, according to the American Osteopathic Association. Their training is similar to that of traditional medical doctors but differs in that they are trained to look at the body and its care holistically rather than just prescribing medicine for specific symptoms or illnesses.

Osteopathic doctors are required to complete additional coursework to understand how the body’s bones, muscles and nerves affect overall functioning and health, and they sometimes use manual techniques for diagnosis and treatment, such as applying pressure to or stretching out joints and muscles. They can also prescribe medicine.

Dr. Conley, who received his bachelor of science degree from the University of Notre Dame in Indiana, has served as an emergency doctor for the U.S. Navy.

A native of Pennsylvania, Dr. Conley completed his residency at the Naval Medical Center in Portsmouth, Va., in 2013. After his residency, Dr. Conley served as chief of trauma for the NATO Role 3 Multinational Medical Unit in Afghanistan.

He served as director of the medical center’s combat trauma research group for a little over two years.

In May, Dr. Conley gained attention after revealing that Mr. Trump had started taking hydroxychloroquine, an anti-malaria drug, while under his care.

Many experts have questioned the drug’s effectiveness in treating, preventing or curing Covid-19 despite claims from Mr. Trump.

The Food and Drug Administration warned in April that it should be used only in clinical trials or in hospitals. The agency also said the drug could cause dangerous heart rhythm problems.

In a letter in May discussing Mr. Trump’s use of hydroxychloroquine, Dr. Conley said he and the president had “concluded the potential benefit from treatment outweighed the relative risks.”

At the news conference on Saturday, Dr. Conley told reporters that Mr. Trump was not taking hydroxychloroquine.

“We discussed it,” Dr. Conley said. “He asked about it. He’s not on it now.”

Dr. Conley was thrust into the national spotlight on Saturday after he gave an upbeat assessment about Mr. Trump that was later contradicted by the White House chief of staff, Mr. Meadows.

Dr. Conley also appeared to indicate that the president had first learned he had the virus on Wednesday rather than late Thursday night. Mr. Trump disclosed the test result on Twitter early Friday morning.

Dr. Conley later released a statement that he misspoke and clarified the timeline of the president’s illness.

Dr. Conley repeatedly said at the news conference that the president was not currently on supplemental oxygen but also declined to say whether Mr. Trump had ever been on oxygen.

Two people close to the White House said in interviews with The New York Times that the president had trouble breathing on Friday and that he received supplemental oxygen at the White House after his oxygen level dropped.

The White House has employed regular physicians since at least 1898, when President William McKinley hired a Navy physician, Presley Rixey. He cared for Mr. McKinley and his wife, said Ludwig M. Deppisch, a medical historian and author of “The White House Physician: A History From Washington to George W. Bush.”

Before then, Dr. Ludwig said, presidents had typically called on military or civilian doctors as needed. But the role of the White House physician as it exists today, a permanent government post for a medical doctor responsible for monitoring the president’s health on a continuing basis, was not enshrined in law by Congress until 1928, by which time the role had become more professionalized, he wrote.

The White House now also has a robust medical unit that includes other doctors and nurses responsible for the well-being of the president, the vice president and White House staff.

Dr. Daniel A. Ruge was the White House physician who made crucial decisions after President Ronald Reagan was shot in 1981. Twelve days after the shooting, Mr. Reagan returned to the White House. Many doctors credited Mr. Reagan’s recovery, if not his survival, to Dr. Ruge’s decision not to disrupt the hospital’s routine.

At other times, the position of White House physician has been complicated by political interests. For instance, Woodrow Wilson’s physician hid from Congress and the American public the severity of the president’s stroke shortly after World War I, which had implications for the nation’s ability to engage in foreign relations.

Concepción de León contributed reporting.