By all accounts, smallpox was the worst pestilence mankind ever faced. The virus, which travels through the air, persisted for 10,000 years. "We're not sure when it started," says Henderson. "But we do have Egyptian mummies that exhibit the actual pustular lesions on the bodies. Ramses V was the most famous one."
When Spaniards carried smallpox to the New World in the 16th century, the virus caused a virtual holocaust among the native populations. In this century, it killed 300 million people, more than AIDS, Spanish influenza and warfare combined.
By 1966, the year the eradication program began, the disease afflicted about 15 million people, hitting India and neighboring countries hardest. Henderson's campaign was vast. Across South America, Africa and Asia, thousands of workers endured heat and mud, assassinations and civil war to deliver vaccine.
Henderson managed the campaign from his office in Geneva, Switzerland, but traveled to trouble spots everywhere. He had a talent for building morale among exhausted field workers. He also welcomed them to his home, where they shared stories from the front -- like the one about a colleague who fell asleep under the stars and was eaten by a lion.
Success came country by country -- first Brazil, then the West African nations, Bangladesh and India later on. To find the last victims, the health workers offered money. When the final case was cornered in Somalia in 1977, the battle was won. Smallpox remains the only infectious disease to be wiped from the face of the Earth.
The campaign succeeded, in part, because Henderson pushed his staff to improvise. They couldn't vaccinate the entire world, but they beat smallpox by isolating the sick and immunizing anyone within a tight radius. The vaccine breaks down in the heat, so they had it freeze-dried. Vaccine supplies were limited, so they invented a two-pronged needle that held the right amount without wasting a drop.
Today, Henderson's home in Guilford is a museum to the eradication program. There are tribal masks from New Guinea, a carved musk ox from India, and wooden statues from Africa -- a man covered with boils, a witch doctor wearing a skull around his neck.
In one corner is an Ethiopian patchwork that depicts scenes from the campaign: a jeep half-submerged in mud, villagers looking up at a helicopter, doctors watching an inoculation. "One of these is D.A.," says Nana Henderson, "but we're not sure which."
Soon after he left the campaign in the late 1970s, the Hopkins School of Public Health courted him to become its dean. He refused, declaring that such schools were "dinosaurs" where researchers thought great thoughts but seldom got their hands dirty.
When someone told him to stop criticizing and get involved, Henderson caved in. He brought a new spirit of activism -- family planning programs in the Third World, studies among intravenous drug users in Baltimore, AIDS programs in the United States and abroad. He held the job about a decade.
He didn't worry about bioterrorism until the mid-1990s. First came the defection of Saddam Hussein's son-in-law, who reported that Iraq's bioweapons program was more sophisticated than anyone had imagined. Then came the lethal gas attack by the Japanese cult Aum Shinrikyo, which also planned to spray anthrax and botulism in Tokyo.
Third, and most frightening, were reports that the former Soviet Union had produced tons of smallpox virus during the Cold War. Scientists there were believed to be experimenting with other infectious agents, too. With the collapse of communism, many scientists left the country hungry, and were said to be peddling their expertise -- and possibly some samples -- to eager bidders.
"Should we be concerned?" says Henderson. "I don't know, but it seems to me that unless we take some steps to protect ourselves, we're kidding ourselves. It would be foolish not to."
Henderson and others say that smallpox poses the most serious threat because it so lethal and so easily spread. Vials of smallpox are kept inside well-guarded freezers at the the U.S. Centers for Disease Control in Atlanta and a virology lab in Russia. These are the last stores sanctioned by WHO, but Henderson says there could be clandestine stockpiles.
"I'd worry about North Korea and Iran, and a little further down the list, Iraq and Libya," he says. "What they're doing with it, I don't know."
With a passion that infuriates some critics, he has called upon the United States and Russia to destroy their stores. "It would create a moral climate in which the use of smallpox would be considered a crime against humanity," he says. Others argue that there is no way to verify that every crackpot who might have the virus would follow the lead.