Scientists believe that the H1N1 form of influenza "A" might not be circulating among humans had it not been for a leak, possibly somewhere in Asia or the Soviet Union, around 1977.
Forms of H1N1 flu circulated widely in the first half of 20th Century, most notably during the devastating 1918 pandemic which killed about 40 million people.
As immunity developed in humans, it was effectively replaced by other varieties of flu. There were no recorded cases of human to human transfer of H1N1 for 20 years from 1957.
Although there were occasional instances of transfer from pigs to humans during that time, it did not circulate between people again until 1977 when there were outbreaks of H1N1 flu in China, Hong Kong and the USSR, when immunity was lower.
A new study of the historical origins of the current outbreak published in the New England Journal of Medicine, suggests that it may only have re-emerged by accident.
"Careful study of the genetic origin of the virus showed that it was closely related to a 1950 strain but dissimilar to influenza A (H1N1) strains from both 1947 and 1957," the paper by Dr Shanta Zimmer and Donald Burke, of the University of Pittsburgh, says.
"This finding suggested that the 1977 outbreak strain had been preserved since 1950.
"The re-emergence was probably an accidental release from a laboratory source in the setting of waning population immunity to H1 and N1 antigens."
Almost 6,000 people have now been infected with swine flu since the outbreak reached Britain two months ago.
Three people infected with the disease in the UK have died, the latest being a young girl in Birmingham who passed away on Friday.
There were also fears that the virus had taken hold at Wimbledon after four ball boys and girls were sent home with suspected swine flu.