An international airline is contacting some of its recent passengers after a man who made seven flights in less than a week was diagnosed on Thursday with the deadly Sars virus.
The man's journey took him from Asia to several European capitals and back to Asia again.
Lufthansa - the airline he used for all seven flights - has been trying to contact passengers and employees who had direct contact with him.
One UK man who had contact with him has been taken into hospital with a probable case of Sars.
The World Health Organisation has said the risk of air passengers spreading the disease mid-flight is extremely low.
But the incident highlights the problems facing health officials attempting to halt the worldwide spread of the virus.
The infected man, who has not been named, started and ended his journey in Hong Kong.
The territory has so far recorded nearly 1,000 cases of the Sars virus and ranks as the most severely affected region after mainland China.
According to the Department of Health in Hong Kong, the 48-year-old victim flew to Munich on 30 March, before travelling on to Barcelona, where he first developed symptoms of the virus.
On 2 April he took another flight to Frankfurt, before moving on to London, back to Munich, back to Frankfurt and then finally returning to Hong Kong on 5 April.
He was then admitted to a hospital in Hong Kong and diagnosed with Sars on Thursday, the Department of Health said.
Lufthansa said it had disinfected all the planes involved and was confident that the chances of any passengers becoming infected were "very remote".
Michael Lameerty, a Lufthansa spokesman, said all the aircraft concerned had efficient filter systems which help remove viruses from the air.
He conceded that the filter systems would not be effective for those sitting right next to an infected individual, but said that "even then, the risk is very low".
Richard Thompson, a spokesman for the WHO, also played down the risks of in-flight infection.
"We've followed thousands of passengers on flights with people infected by Sars, and found very little risk," he said.
There have so far been two people who may have contracted the disease during a flight, Mr Thompson said - although he emphasised that the mode of transmission in both cases was still unclear.
"The overwhelming majority of people who have contracted the virus have become infected through close face-to-face contact," he said.