On April 17, 1957, Maurice Hilleman realized a pandemic was on its way to the United States. That day, The New York Times reported on a large influenza outbreak in Hong Kong. One detail, in particular, caught the doctor’s eye: in the long waiting lines for clinics, the paper said “women carried glassy-eyed children tied to their backs.” He quickly got to work, putting out the word that there was a pandemic coming and pushing to develop a vaccine by the time school started again in the fall.

The first case of the pandemic had appeared in the Guizhou Province of southwestern China in February 1957. By the time Hilleman read about it in April, the Times reported that an estimated 250,000 Hong Kong residents—or 10 percent of the region’s population—were receiving treatment for it.

“We all missed it,” he later recalled for The Vaccine Makers Project. “The military missed it, and the World Health Organization missed it.”

The day after reading the story, he sent a cable to an Army Medical General Laboratory in Zama, Japan, asking the staff to investigate what was going on in Hong Kong. A medical officer identified a member of the U.S. Navy who’d become infected in Hong Kong and sent the serviceman’s saliva back to Hilleman in the United States so he could study the virus.

As the chief of respiratory diseases at the Walter Reed Army Institute of Research in Washington, D.C., Hilleman “had access to large numbers of serum obtained from people at various ages through previous years and decades,” says pediatrician Paul A. Offit, who serves as director of the Vaccine Education Center at the Children’s Hospital of Philadelphia and is the author of Vaccinated: One Man's Quest to Defeat the World's Deadliest Diseases.

In his work at Walter Reed, Hilleman had made the critical observation that the two key proteins in the flu virus—hemagglutinin and neuraminidase—undergo slight changes, or “drift,” between seasons (this insight helped him predict the need for yearly flu vaccinations).

By comparing the Navy serviceman’s virus against previous flu viruses, “what he found was that there was this dramatic shift,” Offit says. “Both those proteins were completely different from what they had been previously. They hadn’t just drifted, they’d shifted.” This new virus was a completely different strain of the flu.

Hilleman couldn’t find any evidence of population immunity to this new strain, so he sent the virus to other health organizations to confirm his findings. These organizations found that the only people who had antibodies to the virus were a small group in their 70s and 80s who had survived the “Russian flu” pandemic in 1889 and 1890.

With this knowledge, Hilleman put out press releases announcing a new flu pandemic had arrived and would reach the United States by September 1957. Though he met some resistance, he successfully convinced companies to begin working on flu vaccines to have ready by then. Fertilized chicken eggs would be necessary for this production, so he told the companies to remind farmers not to kill their roosters at the end of the hatching season.

Doctors and scientists first developed viable flu vaccines in the 1940s, so they were not starting from scratch when they went to work on the 1957 flu vaccine. Still, Hilleman bypassed regulatory agencies in his efforts to push the vaccine forward because he worried those agencies would slow the process down.

When the new flu strain hit the United States in September, just as Hilleman had predicted it would, the country was ready with a vaccine. The virus, dubbed the “Asian flu,” killed an estimated 70,000 to 116,000 Americans and one to four million people worldwide, but experts suggest it would have killed many more if not for the vaccine. Around the time Hilleman was born, the “Spanish flu” of 1918 to 1919 killed an estimated 675,000 Americans and 50 million people worldwide.

“That’s the tricky thing about public health,” says Alexandra Lord, chair of the division of medicine and science at the Smithsonian National Museum of American History in Washington, D.C.

“When things go really, really well, it’s very hard to say, ‘this many lives were saved,’ because we don’t know what would’ve happened without it,” she says. “And so while it’s impossible for us to say how many lives exactly were saved, I think it’s just good to say that he dramatically turned a pandemic around.”

Before his death in 2005, Hilleman helped develop more than 40 vaccines, many of them for childhood diseases. For this work, experts have credited him with saving millions of lives. In 1988, he received the National Medal of Science for his contributions to public health.