Diseases that were routine hazards of childhood for many Americans living today now seem like ancient history.

To a remarkable extent, we owe our well-being, and in many cases our lives, to one man and to events that happened 50 years ago.

At 1 a.m. on March 21, 1963, an intense, irascible but modest Merck scientist named Maurice R. Hilleman was asleep at his home in a Philadelphia suburb when his 5-year-old daughter, Jeryl Lynn, woke him with a sore throat. Dr. Hilleman felt the side of her face and then the telltale swelling beneath the jaw indicating mumps.

For most children, mumps was nothing worse than a painful swelling of the salivary glands. But Dr. Hilleman knew that it could sometimes leave a child deaf or otherwise impaired.

He quickly swabbed the back of Jeryl Lynn's throat, then took the specimen to his laboratory.

Today 95 percent of American children receive the M.M.R. - the vaccine for measles, mumps and rubella that Dr. Hilleman invented, starting with the mumps strain he collected that night. M.M.R. is also used widely in certain other parts of the world.

At Dr. Hilleman's death in 2005, other researchers credited him with having saved more lives than any other scientist in the 20th century. He devised or substantially improved more than 25 vaccines, including 9 of the 14 now routinely recommended for children.

"One person did that!" said Dr. Anthony S. Fauci, a longtime friend of Dr. Hilleman's and now director of the National Institute of Allergy and Infectious Diseases. "Truly amazing."

After getting a Ph.D. in microbiology at the University of Chicago, Dr. Hilleman spent most of his career at Merck, one of the world's largest pharmaceutical companies. Everyone recognized Dr. Hilleman's genius at discovering and perfecting vaccines, which he pursued, Dr. Fauci said, with a rare combination of "exquisite scientific knowledge" and an "amazingly practical get-it-done personality."

Vaccines coax the immune system to resist a disease without producing the actual symptoms, and making them was as much an art as a science. "It's not like there was a formula for this," said Dr. Paul A. Offit, a Philadelphia pediatrician, vaccine developer and the author of "Vaccinated," a 2007 biography of Dr. Hilleman.

In 1963, the United States Food and Drug Administration granted the first license for a vaccine against measles. Much of the early work had been done at Boston Children's Hospital, but the vaccine still commonly produced rashes and fevers when Dr. Hilleman began to work on it.

Dr. Hilleman and Dr. Joseph Stokes, a pediatrician, devised a way to minimize the side effects. It was the beginning of the end of the disease in the United States.

Dr. Hilleman then went on to refine the vaccine over the next four years, eventually producing the strain that is still in use today.

One other crucial event in the development of M.M.R. happened in 1963: An epidemic of rubella began in Europe and quickly swept the globe. In the United States, the virus caused about 11,000 newborns to die. An additional 20,000 suffered birth defects.

Dr. Hilleman was already testing his own vaccine as the epidemic ended in 1965. But he agreed to work instead with a vaccine being developed by federal regulators. By 1969, he had refined it enough to obtain government approval and prevent another rubella epidemic. Finally, in 1971, he put his vaccines for measles, mumps and rubella together to make M.M.R., replacing six shots with just two.

In 1998, The Lancet, a respected British medical journal, published an article alleging that M.M.R. had caused an epidemic of autism. Multiple independent studies would eventually demonstrate that there is no link between M.M.R. and autism, and the original article was retracted.

Jeryl Lynn Hilleman, now a financial consultant to biotech start-ups in Silicon Valley, said that her father was driven "by a need to be of use - of use to people, of use to humanity."

"All I did," she added, "was get sick at the right time, with the right virus, with the right father."

The New York Times