A famous picture from the 1918 flu pandemic shows so many rows of bedridden soldiers that it looks like an optical illusion — the double-mirror effect. It’s a jarring image to accompany jarring events that began in January 1918 and quickly subsumed the planet.

Little about the Spanish flu, which would ultimately infect one-third of all souls on Earth and kill a staggering 50 million, makes sense. Most influenza outbreaks pick off the weakest among us: the young, elderly, and infirm. But this one, called the “greatest medical holocaust in history,” targeted and killed healthy adults. Even today, “its origin remains puzzling,” said a Centers for Disease Control and Prevention study that called it “the mother of all pandemics.”

“Seventy-five years of research has failed to answer the most basic question about the 1918 pandemic: why was it so fatal,” the study said.

The unknowns of this flu virus and others have divided the scientific community. Some researchers think fatal strains should be re-created for analysis. Others think such an endeavor couldn’t be more dangerous. What if something goes wrong? What if an experiment accidentally unleashes a modern pandemic?

The defenders of such research parry: What if an epidemic happens naturally and we find ourselves unprepared?

At the center of the international debate is a thin, intense-looking man named Yoshihiro Kawaoka, a researcher at the University of Wisconsin at Madison, who studies influenza virus and Ebola. On Wednesday, he and an international team of scientists published a study in Cell Host & Microbe that said they created a life-threatening virus that is only 3 percent different from the 1918 Spanish flu, which likely killed more people than the Black Death.

According to Kawaoka’s study, the “contemporary avian influenza virus … may have pandemic potential.” Researchers accomplished its creation with reverse genetics. They stitched together extant bird flu genes to spawn this single new virus that could spread from animal to animal in water droplets.

The stated objective was to “assess the risk of [the] emergence of a pandemic influenza virus” similar to what took out 50 million of us in 1918. “Despite having conquered many infectious diseases, we continue to face a threat from novel, previously unrecognized infections diseases,” Kawaoka wrote. In an e-mail to the Washington Post, he added that “these findings have real world applications.”

Other scientists aren’t buying Kawaoka’s goals. “The work they are doing is absolutely crazy,” Lord May, the United Kingdom’s former chief science adviser, told the Guardian. “The whole thing is exceedingly dangerous. Yes, there is a danger, but it’s not arising from the viruses out there in the animals, it’s arising from the labs of grossly ambitious people.”

A virologist at the renowned Pasteur Institute in Paris was similarly fretful. “It’s madness, folly,” Simon Wain-Hobson explained to the Guardian. “It shows profound lack of respect for the collective decision-making process we’ve always shown in fighting infections. If society, the intelligent lay-person, understood what was going on, they would say, ‘What the F are you doing?'”

What the F he is doing, Kawaoka claims, is trying to alert people to how easily another pandemic could occur. He said the laboratories he works with have top-notch security and the critical researchers “do not understand how highly regulated this work is.”

“This is information for those making decisions about surveillance and pandemic preparedness,” explained Kawaoka, who was ensnared in another controversy in 2012 when the American scientific advisory panel said key details in a study he did on the bird flu should be censored to keep terrorists from creating a biological weapon. (The panel later dropped its objections.)

Such dramas nonetheless lay bare one of the defining questions of this scientific epoch: Just because science can do something, should it? Outbreaks of disease continue to saturate the globe. In West Africa, Ebola has killed more than 220 people in less than three months. In Saudi Arabia, Middle East respiratory syndrome has killed 282 since 2012. Science, some researchers contend, should do what it can to help.

Still, countervailing studies posit the risk of such work is far greater than any potential benefit. “Risk evaluations surrounding biomedical research have not kept pace with scientific innovations in methodology and application,” Marc Lipsitch wrote earlier this year in Plos Medicine.

Even by “conservative estimates,” he said, if 10 labs similar to Kawaoka’s performed high-risk experiments in the next 10 years, it “would run nearly a 20 percent risk of resulting in at least one laboratory-acquired infection, which, in turn, may initiate a chain of transmission.”

Lipsitch expounded on those ideas in an interview with the Guardian. “I am worried that [Kawaoka’s work] signals a growing trend to make transmissible novel viruses willy-nilly, without strong public health rationale. This is a risky activity, even in the safest labs. Scientists should not take such risks without strong evidence that the work could save lives, which this paper does not provide.”