On April 6, 1947, an estimated 1,250,000 New Yorkers were lured by record warm weather to Fifth Avenue for the city’s annual Easter Parade, the largest turnout for the pageant since World War II had ended a year and a half earlier.

On the avenue, women in their Easter bonnets sported colorful spring suits and print dresses. Attention-getters in the crowd included a woman carrying a live Easter bunny wearing a miniature version of its owner’s chapeau. St. Patrick’s Cathedral was abloom with azaleas and dogwoods.

But New Yorkers’ festive post-war mood would soon change dramatically, for a monstrously fatal and highly contagious disease had quietly slipped into the city.

“What happened in New York City was successful because of federal, state, and local communication, voluntary vaccinations, and a public-information blitz–and that’s what’s needed in an any future potential pandemic, or epidemic,” says Judith W. Leavitt, professor of History of Medicine at the University of Wisconsin.

In the coming days, almost five times the number of people who had turned out for the Easter celebration would fill the streets outside city hospitals, police stations and schools to have their skin scratched by a three-inch, two-pronged needle containing a very special vaccine. New York City, which is now grappling with the threat of the H1N1 swine flu pandemic, in the last half-century faced—and conquered—an even greater and more frightening threat: smallpox, which is among the world’s most dreaded infectious diseases.

With the U.S. facing the persistent threat of bioterrorism in the age of 9/11 (remember the anthrax scares) and a presidential panel estimating that H1N1 could infect 50 percent of the country’s population this year, causing some 90,000 deaths and hospitalizing 1.8 million, it is worth remembering how New York dealt with smallpox, which was responsible for 500 million deaths worldwide in the 20th century.

The outbreak began when a 47-year-old businessman, Eugene Le Bar, and his wife stepped off a bus from Mexico City at the midtown Manhattan terminal on March 1, 1947. Le Bar had a headache, sore throat, backache and rash, which he had been treating on the five-day bus trip with “large amounts” of a headache powder and aspirin. Still thinking his condition would improve, the Le Bars decided to spend a few days in the city sightseeing and shopping before continuing on to Maine, where they had a home. Their decision to check into a hotel, window shop, and cruise through a department store mixing with huge crowds would soon have the nation’s most populated city in an uproar.

With his condition worsening, Le Bar was admitted to Bellevue Hospital on March 5. When the rash continued to spread, stymied doctors grew concerned, and on March 8 he was transferred to the city’s communicable disease hospital, Willard Parker.

According to a report in the American Journal of Public Health by Dr. Israel Weinstein, New York City’s Commissioner of Health through the crisis, doctors came up with four possible diagnoses, one of which was that the medications Le Bar had taken aboard the bus had caused a “toxic eruption,” resulting in the spread of his rash, which now consisted of pus-filled blisters. Smallpox, however, was not considered, mainly because the patient had a vaccination scar from childhood.

The doctors were shocked when Le Bar suddenly died on March 10. Because an autopsy disclosed multiple internal hemorrhages, the cause of death was tentatively listed as “hemorrhagic bronchitis.” Meanwhile, there were other patients at Willard Parker with whom Le Bar had either direct or indirect contact—27-year-old Ismael Acosta, with mumps; a 22-month-old girl with croup, and a 2 -year-old boy with whooping cough—who had never received smallpox vaccinations. They recovered from their initial illnesses and went home, according to Weinstein.

Nine days after Le Bar’s death, the girl developed a rash, and was readmitted to the hospital; the diagnosis was chicken pox. A few days later Acosta, who worked at Bellevue Hospital, also broke out in a rash, and was admitted to Bellevue’s dermatology ward; five days later, he was transferred back to Willard Parker with a diagnosis of “possible” chickenpox. As it turned out, neither of the rashes conformed to chicken pox, but rather smallpox, which was later confirmed by physicians at the U.S. Army Medical School Laboratory, and at Western Reserve University. Eugene Le Bar’s case was reevaluated and his skin lesions, indeed, proved to be smallpox.

“It was now evident,” declared Weinstein, “that he had been the source of infection” for the others. Working quietly to avoid panic, doctors at Willard Parker sought out anyone who might have had contact with the victims at the hospital, along with employees and guests at the Le Bars’ midtown hotel. All of those who could be found were vaccinated. Those who rode on the bus with the smallpox carrier, and those in cities where the bus made passenger stops were also sought out. Luckily, no cases were identified.

But the situation worsened in New York and its environs.

On April 6, the same day that throng of New Yorkers turned out for the Easter Parade, Ismael Acosta’s feverish 26-year-old wife was admitted to Willard Parker. She developed a rash on April 7 and died of smallpox on April 12. Simultaneously, three middle-aged men who had been at Bellevue along with Acosta were stricken. The still tiny, but deadly outbreak was also now spreading outside the city. The last major outbreak, in 1875, had claimed the lives of some 2,000 New Yorkers.

On the same day that Le Bar died, a boy of 4 was discharged from Willard Parker and sent to a convalescent home in Millbrook, New York, an hour and a half from Manhattan. While there, he was diagnosed with smallpox, and three others at the home—two children and a 62-year-old nun—caught smallpox from the boy. However, Mrs. Le Bar, who had been successfully vaccinated when she visited her husband at Willard Parker, never contracted the disease.

With the threat of a major epidemic looming, city officials put together an emergency plan for mass inoculations. Newspapers and radio stations alerted the public to be immediately vaccinated. Some 650,000 doses of vaccine were immediately available, and another several hundred thousand units of vaccine was quickly gathered around the country and shipped to New York by the U.S. military. But that still wasn’t enough to inoculate the entire city; millions of doses were required. Time was running out.

New York Mayor William O’Dwyer called an emergency meeting of vaccine manufacturers, who began working around the clock to package and divert vaccine to the city. By this time, thousands of New Yorkers were lining up at hundreds of locations to be inoculated. At the White House, President Truman, who was planning a trip to New York, stood tall for the needle.

Some 500,000 people were vaccinated in the first two weeks, and by the end of April more than 6,350,000 had been immunized. New Yorkers, used to the wartime air-raid drills, had fully cooperated, even waiting in driving rain and lines that went around the block. There was no sense of panic among the population.

As Weinstein pointed out, “Never before had so many people in one city been vaccinated in such a short time and on such short notice,” averting “a major catastrophe…It is little short of remarkable that there were only 12 cases [and two deaths] in the entire outbreak.” Without the mass action, he declared, “there very likely would have been thousands of cases [of smallpox] and hundreds of deaths.”

“What happened in New York City was successful because of federal, state, and local communication, voluntary vaccinations, and a public information blitz—and that’s what’s needed in an any future potential pandemic, or epidemic,” asserts Judith W. Leavitt, professor of history of medicine at the University of Wisconsin. Leavitt, who has written and lectured about New York’s smallpox scare, told me that “strong, well-funded local and state health departments are also a prerequisite to success,” but she’s concerned that today “health departments have been poorly funded.”

While the H1N1 virus looks for now to be non-lethal for most victims, the smallpox virus—eradicated from the general population but reportedly stored at the Centers for Disease Control and Prevention in Atlanta, and in a lab in Siberia—is still a serious threat if placed in the hands of bioterrorists.

Veteran journalist Jerry Oppenheimer is the author of a biography of Ethel Kennedy, The Other Mrs. Kennedy: An American Drama of Power, Privilege, and Politics. His most recent book, Madoff With the Money, was excerpted in The Daily Beast.