The outbreak of hepatitis A on Statoil's Statfjord C platform was caused by a person carrying the disease who passed it on via food.
This finding has been made with help from the Norwegian Food Control Authority, the National Board of Health and infectious disease specialists at the Rogaland central hospital in Stavanger.
Nine people on the North Sea installation taken ill during the outbreak were probably infected between 18-23 May, but none of them was the original carrier.
Nor do any of the catering staff on the platform appear to have been the source of the infection.
"We believe the outbreak was caused by a single carrier," reports Reidunn Ulland von Brandis, chief medical officer for Statfjord.
"This person could be anyone who was on the platform during the period in question."
Dr von Brandis believes that the nine who caught the disease have eaten from a dish which had been in contact with the carrier, which is a normal route of transmission.
While confirming that the outbreak was confined to the nine infected in May, she warns that another two-three people could end up with the disease.
But she discounts the possibility of a new outbreak involving a large number of people.
Although it is impossible to guard against people who neglect hygiene precautions, Dr von Brandis emphasises the importance of informing newcomers on the platforms about existing routines.
Everyone offshore is required to observe rules on personal hygiene and to wash their hands after using the toilet.
"It's important for Statoil to ensure that its own employees, contractor personnel and newcomers are notified of these requirements," says Dr von Brandis.