Ten ferrets, some bird flu and swabs. That is all, according to a presentation last summer, one needs to concoct a virulent strain of influenza that could start a deadly pandemic among humans.

These initial findings were presented last September in Malta at the European Scientific Working group on Influenza meeting to an auditorium packed with fellow scientists and policy makers. Ron Fouchier, a virologist at Erasmus Medical Center in Rotterdam, himself a bit sniffly at the time, calmly explained that he and his team had discovered that without the help of another virus, the deadly avian flu (H5N1) could easily mutate in mammals to become transmissible through the air, like a true pandemic strain, through a sneeze or a cough. And it might need as few as five mutations to make that leap.

The findings took time to gain attention wider public attention, but when research manuscripts describing the specific mutations that the virus underwent were submitted to scientific journals, many scientists and commentators protested, suggesting such information could be used by bioterrorists to manufacture a pandemic flu strain. But after extensive review by researchers, journal editors and even the National Science Advisory Board for Biosecurity, a decision was finally made to take the papers public.

Today in Science, the full paper on the ferret experiment is being published and made freely available online in its entirety. This publication follows the May release of a similar H5N1 study in Nature. (Scientific American is part of Nature Publishing Group.) A second Science study, also published online June 21, assesses the likelihood that these mutations will occur in nature and spur a pandemic in humans. Today's paper is published alongside eight essays explaining some of the risks and benefits of this sort of research.

The upshot is further evidence that this deadly bird flu spreading to humans "is absolutely in the realm of possibility," Derek Smith, of the University of Cambridge, who co-authored the separate, second Science paper said at a Wednesday press briefing. "We see no fundamental hurdle to that happening."

H5N1 has already been found in some 20 mammalian species, including dogs, cats, humans and the dreaded virus reservoir, pigs, Fouchier noted at the briefing. About 600 human deaths have been confirmed in the past 15 years as being caused by this infection, acquired directly from birds. (The mortality rate among humans has been reported at a scary 60 percent, but this is likely an overestimate because non-lethal cases are probably not reported as often, Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, and Francis Collins, director of the National Institutes of Health, noted in an essay in the same issue of Science)

"Whether this virus may acquire the ability to be transmitted via aerosols or respiratory droplets among mammals, including humans, to trigger a future pandemic is a key question for pandemic preparedness," Fouchier and his colleagues wrote in their paper.

The researchers started with a strain called influenza virus A/Indonesia/5/2005, which has infected and killed a number of humans. They first changed three proteins to make it more compatible with mammals' tissues and body temperatures. And then they introduced it into the nose of a ferret.

Ferrets are the best model we have for studying influenza—they are similarly susceptible to it and can spread it by sneezing—and have been used to study the infection since the 1930s. But that does not mean that they are a perfect flu foil for humans. "An H5N1 virus strictly adapted for ferret transmissibility may not be entirely relevant to humans," Fauci and Collins wrote in their essay.

The experimental ferrets Fouchier used did not pass the disease directly to one another, as would occur among humans in a real-world outbreak. Instead, during the first six of the 10 transmissions, researchers dropped the strain into the ferret's nose and then later swabbed the nasal area to collect the virus—with any new mutations—to be given to the next ferret. In the final few transmissions, instead of swabbing the nose, researchers made each animal sneeze, then collected the contents and placed them directly in the nose of the next ferret. Follow-up experiments showed that when an uninfected ferret was placed near one of these infected subjects—sharing air but not able to make physical contact—the uninfected ferret often caught the illness (looking tired and not eating much) but did not die from it.

As Fouchier and his co-authors noted, none of the ferrets that received the newly transmissible virus died from the infection. (And only one in eight of the animals given an earlier form of the virus, via the nose inoculation, died from the illness.) This shift in virulence mirrors the pattern that has often played out in human pandemics: as a virus becomes more easily transmissible, it becomes less deadly. In the experiments, the viruses were also treatable with common antiviral drugs.

What were the mutations that made the virus able to infect other ferrets just by breathing the same air? Four of the five mutations changed the virus's surface protein (hemagglutinin), which plays a role in allowing H5N1 to gain entry into host cells. And the fifth mutation boosted its ability to reproduce its genetic information.

Two of the mutations identified by Fouchier are already quite common in wild strains of H5N1 in birds and occasionally show up in the same strain, according to Smith's study of some 4,000 strains. As Smith and his colleagues pointed out in their paper, that means that some strains might need only three more mutations to become easily contagious among mammals. These three other mutations are rare in H5 strains (although two of them were present in the H2 and H3 pandemics of 1957 and 1968, respectively), so they are likely harmful to the virus while it resides in its bird host. Smith and his team created mathematical models to try to figure out how likely it was that these three mutations could occur in a human host. But without further research, they could only speculate that it was possible, they noted.

Trying to assess the likelihood of an outbreak of this virus in humans is about as easy as "predict[ing] an earthquake of tsunami," Smith said at the press briefing. "We now know that we're living on a fault line," thanks to Fouchier's research. "And it's an active fault line."

That prediction problem, however, suggests that much more research needs to be done on just how these viruses mutate within hosts. They also suggest increasing surveillance to do deeper genetic sequencing of circulating strains to look for less common mutations that might increase the odds the virus will jump to a new species host.

With a genetic profile of these likely mutations, efforts to monitor for the virus in the wild could be boosted. Such a profile could also help governments and vaccine manufactures get a head start developing a preventive shot before a pandemic has a chance to gain speed.

As Fauci pointed out in the press briefing, this sort of research should be closely watched and, indeed, debated. "The risk-benefit calculation is not always obvious," he said. But despite the risks—that bioterrorist groups could try to replicate the strain or that it could accidentally escape the laboratory—he sees the research as necessary so that we may better understand how the virus might evolve to infect humans. The ferret research was conducted at a biosafety level 3 (one step down from the facilities where researchers study Ebola and smallpox).

After controversy over these papers erupted late last year, a voluntary, ongoing moratorium went into effect on all the experiments that increased the transmissibility and/or pathogenesis of H5N1, Fauci noted, adding: "I think the benefit that will come out of the Fouchier paper in stimulating thoughts and ways to better understand transmissibility and adaptability and pathogenesis, in my mind, far outweigh the risk of nefarious use of this information."