Health experts have begun to destroy samples of a potentially lethal flu strain sent to laboratories around the world by a US testing organisation.
Current vaccines do not guard against the 50-year-old flu strain
The samples are of Asian flu, which killed between one and four million people in 1957 but disappeared by 1968.
Testing kits containing the virus were sent to more than 3,700 laboratories in 18 countries from Brazil to Lebanon.
The World Health Organization said the virus could "easily cause an influenza epidemic" if not handled properly.
Canada, Hong Kong, South Korea and Singapore have all destroyed their samples, the WHO said, and Germany said it had disposed of its virus as well.
Taiwan was said to be moving "very, very fast" to destroy the samples it had received, and Japan said its health ministry had ordered it destroyed in the nine labs that had it.
The WHO was unable to confirm how much had been destroyed in US labs, which received the vast majority of the samples.
The full list of countries and areas where laboratories received the virus is: Bermuda, Belgium, Brazil, Canada, Chile, France, Germany, Hong Kong, Israel, Italy, Japan, Lebanon, Mexico, Saudi Arabia, Singapore, South Korea, Taiwan and the US.
Because the virus has not been in circulation since 1968, people born after that do not have antibodies against it - and current vaccines do not guard against it.
"If this virus were to infect one person, it would spread very rapidly," Klaus Stohr of the WHO told the BBC.
WHERE THE VIRUS WENT
Europe: Belgium, France, Germany, Italy
Americas: Bermuda, Brazil, Canada, Chile, Mexico, the US
Asia: Hong Kong, Japan, Singapore, South Korea, Taiwan
Middle East: Israel, Lebanon, Saudi Arabia
The College of American Pathologists sent out kits between October 2004 and February of this year.
On 8 April, the US government asked the body to write to the laboratories affected - of which 61 are outside the US and Canada - telling them to destroy the samples.
Given the concerns that the virus could be used in bio-terrorism, letters were sent to the laboratories before the mistake was made public.
Dr Stohr said the College of American Pathologists had not violated US regulations, which are now being revised.
The virus - technically known as H2N2 - was classified as Biological Safety Level 2, meaning that it was not considered particularly dangerous.
But the US government agency responsible for classifying viruses, the Centers for Disease Control and Prevention, says it was in the process of deciding whether to change the strain's classification when it found out that it had been widely circulated.
Labs use the samples to test their ability to make diagnoses
The WHO says there is no guarantee that every sample of the virus can be traced and destroyed because some of the laboratories may have sent derivatives of the sample elsewhere.
But there have been no reports of anyone becoming ill from handling the virus, which the WHO called reassuring.
"The risk is considered to be low... but as long as this is out, it is possible laboratory technicians can become infected," Dr Stohr said.
Laboratories use the kits to show they can correctly identify different strains of a virus.
They normally include strains in current or recent circulation.
It is hoped the laboratories will have destroyed the vials by the end of the week.