Controversial research into making bird flu easier to spread in people is to resume after a year-long pause.
Some argue the research is essential for understanding how viruses spread and could be used to prevent deadly pandemics killing millions of people.
Research was stopped amid fierce debate including concerns about modified viruses escaping the laboratory or being used for terrorism.
The moratorium gave authorities time to fully assess the safety of the studies.
A type of bird flu known as H5N1 is deadly and has killed about half the people who have been infected.
It has not caused millions of deaths around the world because it lacks the ability to spread from one person to another. Cases tend to come from close contact with infected birds.
Scientists at the Erasmus University in the Netherlands and the University of Wisconsin-Madison in the US discovered it would take between five and nine mutations in the virus' genetic code to allow it to start a deadly pandemic.
Dangerous science?
Their research was the beginning of a long-running furore involving scientists, governments and publishers of scientific research.
It is easy to see why designing a more dangerous version of H5N1 would raise concerns. A virus which can kill half of the people it infects and could spread rapidly from person to person is the stuff of Hollywood disaster movies.
However, the research could reveal important insights that could prevent such an infection arising in the wild and help build defences just in case.
The studies in ferrets showed that five to nine mutations were needed to get H5N1 spreading through the air from animal to animal.
This helps health officials tracking the virus as they can keep an eye out for danger signs in the virus' genetic code. Two of the mutations have been seen in the wild - but alone are not enough to set alarm bells ringing.
Getting an idea of what a highly infectious H5N1 virus would look like can also be used to help design effective vaccines and anti-viral medications.
This controversy is about balancing risks - do you study the virus with a remote chance of it getting out of the laboratory or do you avoid such research and miss out on discoveries which could save lives in the next pandemic?
The US National Science Advisory Board for Biosecurity asked academic journals not to publish key parts of the findings. It was concerned terrorists would use the details to develop a biological weapon.
It provoked outcry among some scientists who said their academic freedom was being restricted. Other scientists said the risk of the virus spreading was too great for such research to take place and described it as a folly.
The details were eventually published in the journals Nature and Science.
However, the academics involved agreed to a voluntary 60-day moratorium on research - which was later extended to more than a year.
It was to give governments time to review safety standards needed in laboratories to conduct research with enhanced viruses and whether they wanted to fund such research.
Back on
A letter signed by 40 virus researchers around the world, published in the journals Science and Nature, said the moratorium was being lifted.
It said appropriate conditions had been set in most of the world and their studies were "essential for pandemic preparedness".
One of the leading proponents of the research Prof Ron Fouchier, from the Erasmus Medical Centre, told the BBC it had been "frustrating" to shut down research for the year.
"This research is urgent, while we are having this pause bird flu virus continues to evolve in nature and we need to continue this research.
"We cannot wait for another year or two years."
He expects to restart his laboratory's work within the next couple of weeks.
However, it is a different case for many of the other research groups involved. The US has not decided on the conditions that it will allow the experiments to take place and the same applies to US-funded research taking place in other countries.
The decision has continued debate on whether the research should take place at all.
Prof Robert May, from the University of Oxford and a former president of The Royal Society, said: "These are not bad people, they are good people with good intentions, but they look through rose-coloured glasses at the security of the laboratories."
He said past history suggests "it will get out" as there had been more than a thousand cases of people being infected in labs with the highest standards and the 1977 outbreak of flu may have been connected to a Russian facility.
"That's why I feel the world is a safer place if we maintain this moratorium."
