Authorities on the French Indian Ocean island of Reunion say three tourists who returned from a trip to Thailand may have contracted bird flu.
Tests have been carried out on other members of the tour group
A 43-year-old man was admitted to hospital in Saint-Denis on Saturday complaining of weakness and severe headaches. He later developed a cough.
Two people who travelled with the man are also suspected of having bird flu.
Samples will be tested in Paris for the H5N1 virus, which has killed at least 60 people in Asia since 2003.
The authorities say the three were part of a group of 20 who had been to Thailand.
The three suspect cases visited a bird park during their trip there and had close contacts with birds.
A senior Thai official rejected the suggestion that the bird park might be the source of infection.
"It is impossible that they could have been infected just because they went to the zoo," said Charal Trinvuthipong, assistant to the agriculture minister.
French Health Minister Xavier Bertrand said initial tests carried out on Reunion had been positive, and samples were being rushed to Paris for further tests.
"We will have results for the first patient tomorrow," he told the AFP news agency on Wednesday evening, adding that "for the moment these are only suspicions. Nothing has been confirmed".
Earlier, the EU confirmed H5N1 had been found in wild swans in Croatia.
Officials are bracing for an outbreak of the lethal strain of bird flu, also known as avian flu, in the European Union, after it was recently discovered in neighbouring Turkey, Romania and Russia.
European countries have been taking steps to combat the possible spread of the virus between birds.
Croatian authorities followed the lead of Austria, Switzerland and Germany in asking poultry producers to keep their animals indoors.
On Tuesday, the EU announced that it would ban imports of live wild birds and impose stricter rules on the private ownership of pet birds, after a parrot died of the H5N1 strain while in quarantine in the UK.
As the ban was announced, German officials said two geese had tested positive for the flu in initial checks.
Further tests are to be carried out to confirm the virus and to establish whether it is the deadly strain.
In Asia, China has reported its third major outbreak of bird flu in two weeks.
BIRD FLU OUTBREAKS IN 2005 (H5N1 STRAIN)
The H5N1 strain remained largely in South-East Asia until this summer, when Russia and Kazakhstan both reported outbreaks
Scientists fear it may be carried by migrating birds to Europe and Africa but say it is hard to prove a direct link with bird migration
UK case discovered in quarantine, so disease-free status unaffected