The US government has told more than 3,700 laboratories in 18 countries to destroy potentially lethal influenza samples sent out in testing kits.
The samples are of "Asian flu", which killed between one and four million people in 1957 but disappeared by 1968.
If the virus is not handled properly, "it can easily cause an influenza epidemic", Klaus Stohr of the World Health Organization (WHO) warned.
The virus was sent to Europe, Asia, the Middle East and South America.
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.
Hong Kong radio announced on Wednesday that the virus had been destroyed there.
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," Dr Stohr, the WHO's influenza expert, told the BBC.
The College of American Pathologists sent out kits containing 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.
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
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.
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.