Hong Kong officials on Sunday confirmed the third case of swine flu in the city, and said the patient was a man who arrived from the United States a day ago.
The 23-year-old patient was a mainland Chinese university student who boarded a codeshare flight operated by Cathay Pacific and American Airlines, CX 831/AA 6091, in New York on Friday and arrived in Hong Kong around 1100 GMT on Saturday.
The patient was admitted to hospital and tested for the A(H1N1) virus on Friday night after thermal screening at the airport showed he had a body temperature of more than 38 degrees Celsius, Thomas Tsang, controller of the city's Centre for Health Protection, told a press conference.
"The test came out on Sunday and the test result is positive," he said.
Tsang said the man had developed a fever while on the plane.
He said authorities are now tracing passengers who were sat near the patient on the flight.
Cathay Pacific said in a statement that 37 of the 63 passengers who shared the same cabin zone with the patient had landed in Hong Kong, while the rest have since left for destinations including Manila, Shanghai, Beijing, Taipei and Bangkok.
The case came only days after a 24-year-old Hong Kong man returning from San Francisco tested positive for the flu, and two weeks after officials confirmed Asia's first case of the A(H1N1) virus, a Mexican man, who arrived in the city.