"A less strong person would have bent under the pressure," said Dr. Anthony Fauci of the National Institutes of Health, who taught Koop what was known about AIDS during quiet after-hours talks in the early 1980s and became a close friend. "He was driven by what's the right thing to do."
"He spoke truth to power," added Dr. Georges Benjamin of the American Public Health Association. He called Koop "a national hero."
Koop died Monday at age 96. Here are highlights of his career:
—Koop helped pioneer the field of pediatric surgery in the late 1940s, a time when medicine didn't recognize that children weren't just miniature adults but required special procedures. He established the nation's first intensive care unit for newborns undergoing surgery, and in 1977 he gained international attention for separating conjoined twins.
—Koop became surgeon general in 1981, the same year that the first cases of what would become known as AIDS were discovered.
—In 1984, Koop launched a campaign to make America smoke-free by the year 2000. He stressed the dangers of second-hand smoke. A former pipe smoker, he said cigarettes were as addictive as heroin and cocaine.
"Cigarettes are the most important individual health risk in this country, responsible for more premature deaths and disability than any other known agent," Koop said.
—The conservative Reagan administration was reluctant to address AIDS, and Koop was kept silent on the issue during his first few years in office. But Koop lived on the NIH campus at the time and would stop in Fauci's office on the way home, to ask about the latest science on what he recognized as a rapidly growing threat, Fauci recalled Monday.
"We did that for weeks and weeks until he was conversant enough in it where he said, 'We have got to do something about it. We are too silent. The government is too silent,'" Fauci said.
In 1986, asked by President Ronald Reagan to prepare a report on AIDS, Koop issued the first explicit federal advice on how Americans could protect themselves. He urged the use of condoms for safe sex and advocated sex education as early as third grade, a discussion surprisingly frank for the time.
—In 1988, after some maneuvering around his reluctant bosses, Koop went a step further and sent an educational AIDS pamphlet with that same plain-spoken information to more than 100 million U.S. households.
"He really changed the national conversation," said Chris Collins of amFAR, the Foundation for AIDS Research.
A biography posted online by the National Library of Medicine says that Koop's "report, speeches and television appearances did much to change the public debate on AIDS in the United States and, along with it, attitudes toward public discussion of sexuality."
—Koop resigned as surgeon general in 1989, but continued to lecture widely on a range of public health issues for the next two decades.
In a 2010 speech, he called AIDS the "forgotten epidemic," and urged Americans to end complacency that he called "as dangerous as the irrational fear in the early days of the AIDS controversy."