S. Ward Casscells III was in his early 50s and in the throes of prostate cancer when he told his wife that he had decided to join the Army Reserve. “ ‘This is one heck of a midlife crisis,’ ” he later recalled her saying. “ ‘But in fairness, I realize you can’t afford a Maserati, and you don’t have the nerve to take a mistress.’ ”

Then a prominent cardiologist in Houston, Dr. Casscells had recently shown their young son the military uniform that his own father wore while serving in World War II. Dr. Casscells was “filled with shame,” he told a friend, former Newsweek editor Evan Thomas, when he confessed to the boy that he had not likewise served his country in uniform.

And so it was that Dr. Casscells, who died Oct. 14 at his home in Washington, became Col. Casscells. He joined the Army Reserve in the summer of 2005 and volunteered the following year for duty in Iraq.

There he served as medical liaison to Gen. George W. Casey Jr., the top U.S. military commander in the country, and Zalmay Khalilzad, the U.S. ambassador. Dr. Casscells also worked with Iraqi doctors to help rebuild the local medical community.

“He helped us a lot to focus on increasing the capacity of the Iraqis to take care of themselves,” Khalilzad said in an interview.

“There was a lot of credibility to what he said,” the former ambassador added, “because he had volunteered and because he was such a distinguished person.”

During his four-month tour of duty, Dr. Casscells survived a shelling and an ambush by insurgents. Widely admired, both for his military service and his medical work in Houston, he returned home and assumed a civilian post as assistant secretary of defense for health affairs — effectively the Pentagon official in charge of the sprawling bureaucracy of military medical policies and programs.

His death, from prostate cancer, was confirmed by his wife, Roxanne. He was 60.

Dr. Casscells arrived in Washington in April 2007 at an especially traumatic time for the military. Shortly before President George W. Bush introduced Dr. Casscells as his nominee for the Pentagon job, The Washington Post published the first in a series of articles about the mistreatment of wounded veterans at Walter Reed Army Medical Center in the District.

That series, for which The Post received the 2008 Pulitzer Prize for Public Service, prompted widespread outrage and helped bring about reforms in the military medical system.

“There was no greater advocate for the value that military medicine provides this nation than Dr. Casscells,” Jonathan Woodson, the current assistant secretary of defense for health affairs, said in a statement. “Dr. Casscells was relentless in working to make our system more open, more transparent and more accessible to the Service members, military families and retirees who use it.”

Traditionally, former Pentagon press secretary Geoff Morrell said in an interview, civilians in Dr. Casscells’s position had been medical administrators. He brought to the job the hands-on sensibility of a physician and at times even carried a stethoscope and medical bag when he made rounds across the country and in far-flung parts of the world.

Dr. Casscells became known for “walking the deck,” as he described his technique — dropping by unannounced at Walter Reed and what was then the National Naval Medical Center in Bethesda to observe medical care and visit with troops.

His experience in Iraq, Dr. Casscells once said, left him with a distaste for bureaucracy. He spoke admiringly of a nurse who, after Dr. Casscells was injured in a Humvee accident, kicked him out of a hospital bed so that a more seriously wounded soldier could use it.

“She said, ‘What is that colonel doing? Get him out of here and treat him in the hall,’ ” Dr. Casscells recalled in an interview with the Houston Chronicle. “There was no military medical person more senior than me over there, and here this nurse was throwing me out into the hall.. . . I think that gives you some idea of the focus and dedication.”

In 2009, he published a book titled “When It Mattered Most: Remembering Our Fallen Medical Personnel in Iraq-Afghanistan.”

Samuel Ward Casscells III was born March 18, 1952, in Wilmington, Del. He was named for his father, who helped create the field of arthroscopic surgery. After graduating from Yale University in 1974, Dr. Casscells received a medical degree from Harvard University in 1979.

Survivors include his wife of 22 years, Roxanne Bell Casscells of Washington and Upperville, in Fauquier County; three children, Samuel W. Casscells IV, Henry W. Casscells and Lillian B. Casscells, all of Washington and Upperville; a brother; and two sisters.

Dr. Casscells spent most of his career in Houston, where he became a professor and vice president at the University of Texas Health Science Center, chief of cardiology at the school’s teaching hospital and scholar at the Texas Heart Institute. He was credited with conducting significant research that linked heart attacks and the flu, and he later developed a specialty in avian flu.

Before joining the military, he did humanitarian work in Asia after the tsunami of December 2004 and in Pakistan after an earthquake.

“Some people as they get older get more conservative,” he told the Chronicle. “For whatever reason, I just seem to be getting more adventurous.”