TOKYO, May 4 -- Armed with thermographic guns, Japanese health inspectors in surgical gowns, goggles and masks boarded United Airlines Flight 803 from Washington Dulles. They prowled the aisles, pointing their fever-seeking machines at jet-lagged faces.

The nonstop flight had taken 13 1/2 hours. Toddlers were crying. Adults were wilting. Everyone was under strict orders to stay in his or her seat.

Exhausted-looking flight attendants handed out surgical masks, gifts from the government of Japan, which has yet to find a single confirmed case of swine flu but is diligently seeking feverish suspects.

Passengers could not leave the aircraft until they had filled out a form the government had hurriedly printed. "The Questionnaire of Heath [sic] Status" asked whether travelers had been to Mexico lately, whether they had a runny nose, whether they were taking medication to mask a fever.

As long as the threat of a flu pandemic persists, anyone who flies into this country from North America with flulike ailments will not be allowed to walk off an airplane and infect the country. Last week, inspectors began boarding every flight from Mexico, Canada and the United States. They take the temperature of about 6,000 passengers a day. Near Tokyo's Narita airport, 500 rooms have been secured by the Health Ministry to quarantine infected passengers.

Asia was stung in 2003 by an outbreak of severe acute respiratory syndrome (SARS), which killed about 800 people and caused temporary harm to the economies of Hong Kong, Singapore, China and Malaysia. As a result, governments and health bureaucracies across the region are ready and willing to move aggressively against swine flu.

China suspended flights from Mexico on Saturday, after the first confirmed case of the virus was found in Hong Kong. At the Hong Kong hotel where the swine flu victim stayed, about 200 guests and 100 workers were confined to the premises for a week. In South Korea, which has two probable cases of swine flu, all passengers pass in front of thermographic cameras. Those found to be feverish are held for testing that takes about six hours.

Even though it has yet to find one confirmed case of swine flu, Japan has opened 684 "fever clinics" across the country. Officials installed thermographic imaging devices at the world table tennis championships in Yokohama after a local high school student was admitted to a hospital with what turned out to be a seasonal strain of the flu. On Monday, a woman who had just returned to Narita from the United States tested positive on the plane for influenza A, but further tests found she did not have the swine flu virus.

"It's not a short-term fight, and we need to brace ourselves for what will likely take a considerable time," Prime Minister Taro Aso told reporters Friday.

For jumbo jets arriving from North America, a shortage of health inspectors has meant that considerable time is being spent by passengers in parked airplanes. Thousands of travelers have waited for hours in their seats before inspectors could clear them to pass through immigration.

"We're just about managing to handle the situation with a limited number of inspectors," a government official told the Yomiuri newspaper. "But I wonder what will happen if more outbreaks occur in other countries."

Inspectors boarded United Flight 803 a few minutes after it landed at Narita. They completed their work in 70 minutes. Although everyone was sick of sitting in the airplane, no one was found to be feverish.