Health officials are asking 290 people who came into contact with a Covid-19 patient in Krabi to closely monitor their health as 79 of them are considered "high-risk".
Although most of the man's relatives have tested negative, his wife's status is still unknown.
The concern comes amid public health investigations in Krabi, Phuket, Chiang Mai and Sukhothai, all centres which the man visited recently.
Walairat Chaifu, chief of the Epidemiology Division at the Department of Disease Control (DDC), said yesterday the 37-year-old man, an Indian national who lives on Koh Phi Phi, had twice tested positive since Nov 4, when he underwent a health check while applying for a work permit.
He is now being kept in isolation in a negative pressure room at Krabi Hospital and health officials are waiting for the result of his virus's genetic decode to see if it is a highly spreadable version.
Although a Covid test he gave on Nov 6 was negative, that could be related to the fact that the positive test he'd given just two days earlier showed only small amount of the virus and because he had contracted the disease a long time previously. An immunity check also showed he had been infected for 1-2 weeks or longer, the doctor said.
"The infected person came to Thailand in April 2019, then went abroad in October before returning to Thailand in February and has not been anywhere abroad ever since. He opened a restaurant with his wife on Koh Phi Phi," Dr Walairat said.
Medical officers are checking on his movements and trying to trace those with whom he may have been in contact. From Oct 19-27, he was only on the island. On Oct 28, however, he went to Krabi by ferry and stayed with his brother's family.
On Nov 29, he travelled by car to Phuket and stayed at a hotel in Patong and met an American friend, and had a meal at a restaurant beside the hotel, Dr Walairat said.
On Oct 30, he went by taxi to Phuket Airport and flew to Chiang Mai, where he took another taxi to a city centre night entertainment venue. Both taxi drivers and the bar patrons are considered at risk.
The next day, he went to Sukhothai, where he attended a Loy Kratong event, but other visitors are considered to be at lower risk of infection because everyone had their temperatures taken and face masks were compulsory. "We have made appointments for those in the high-risk group to take the test and self-quarantine for 14 days," Dr Walairat said. She considered medical staff to be among those at highest risk of infection.