On the evening of May 2, he boarded the airline's flight HU7801 to do the cleaning work, as usual, but wearing protective gloves and a mask. The plane had just arrived after a three-hour flight from Beijing.

The next day, the airline's Guangzhou office asked Xiao Su to go to hospital for a medical check-up because one passenger from the flight was suspected of having been infected with SARS. The passenger already had a fever and was flying to Guangzhou from North China's Inner Mongolia with a stop-over in Beijing. She admitted only the next day that her father died of SARS and her mother was in a critical condition with SARS in Inner Mongolia.

Her revelation immediately set the local Guangzhou Center for Disease Control in motion.

By May 9, the Guangzhou center had contacted 44 passengers from the flight, many of whom entered voluntary quarantine at home and promised to go to hospital if they felt unwell, according to Wang Ming, the center's director. By yesterday, no one had developed any SARS symptoms.

Xiao Su, the flight crew and 26 other airport staff were placed under observation.

For Wang Ming and his colleagues across the country, such work, though hard, is now routine because it has been proved effective when it comes to cutting the chain of SARS infection.

Major local and national news media and websites frequently run notices from different provincial health authorities, warning passengers from certain trains, long-distance buses and airlines that they should report to their local hospital for a medical check-up.

When one patient is confirmed as being infected with SARS, doctors call epidemiologists over to carefully interview the patient to find out his or her case history and the people with whom he or she might have come in "close contact."

The work to stop SARS spreading is being firmly established at the rural grassroots level. Researchers have already determined that SARS is easily spread among family members or people in close contact.

On April 29, Lou Zongxin and his wife Xu Zufeng arrived in Wuhu railway station in East China's Anhui Province, after finishing some business they had to do in Beijing. Before they left the station, they had their temperatures taken. That was the second time they had been required to do it on their way home from Beijing.

During the trip between the city of Wuhu and the village of Bafen, a distance of some 50 kilometers, the couple had a third medical check-up before they reached their house. Before boarding the train, they told Lou's parents by phone to empty the family tool shed so that the couple could stay in the shed for two weeks under voluntary quarantine.

Meanwhile, Xu Huacun, the village head, leads the village medical workers every day as they give medical check-ups to Lou and 30 other villagers who came back from SARS-hit areas. "It's for the good of other villagers," said Xu, who heads a village of 776 households and 2,645 residents, with 790 villagers working in big cities outside the province.

Epidemiologists are also checking on possible high-risk groups. For instance, epidemiologists at Peking University have been monitoring those who have chronic illnesses or had visited their hospitalized parents or other relatives during the period between March and early April, when the SARS infection was especially serious in Beijing's hospitals.

All this work has so far contributed to the steady decline in SARS infections in the hardest-hit areas of North China, including Beijing, and to a much lesser incidence in cities and provinces in East and Central China.

All the present-day epidemiology work, now outlined in a Ministry of Health code for SARS prevention, has drawn on the lessons and experiences of earlier work done in Guangdong, North China's Shanxi Province and Beijing. Chuo Ruiyi, director of the disease-control center in Qingxu County in Shanxi, described the work as very hard because the staff could not tell the public the full story and thus win public understanding.

Chuo Ruiyi first heard about feidian (atypical pneumonia) in early February. At that time, "atypical pneumonia" was raging in south China's Guangdong Province and caused panic buying of vinegar there.

Qingxu County, where the famous Shanxi vinegar is produced, saw quite a number of business people from Guangdong arrive and then leave after ordering locally produced vinegar in bulk. "No one in Qingxu was infected in those days," Chuo said.

The first case that alerted the epidemiologists in Shanxi was a businesswoman surnamed Yu, who ran a jewellery store in a shopping center in Taiyuan, the provincial capital.

She was a link in the first SARS infection chain between Beijing and Shanxi. The province has become the third hardest-hit area on the Chinese mainland.

Shanxi has one of the lowest per capita incomes in the country, with a far lower gross domestic product than Beijing and Guangdong. But local medical staff and epidemiologists have been waging a staunch battle.

First Chain

Yu went to Guangdong at the end of February to replenish her shop's stock. She caught a cold and had a fever. With little knowledge of the deadly epidemic as a result of infrequent media coverage of SARS, however, Yu did not go to see a doctor in Guangdong. Instead, she returned to Taiyuan with her stock and lived with her family even though her illness was getting worse.

She visited several hospitals in the city for treatment in late February, but her condition did not improve. Medical staff in the province had no knowledge of the killer disease and had not adopted any preventive measures in the region.

Following the custom of local rich families, she hired a taxi and went to Beijing on March 1 to see a doctor at Army Hospital No 301.

It was not until March 7 that Yu was confirmed as a SARS case and was transferred to Army Hospital No 302, which specializes in infectious diseases. By then, eight of her relatives, including her husband and parents, had already developed symptoms similar to hers. They also came to Beijing for treatment.

Yu became the first SARS patient in Beijing and health workers at Army Hospital No 301 were directly infected.

Shanxi epidemiologists started work at once after hearing of Yu's diagnosis. Zhao Baoxin, the Taiyuan Center for Disease Control, arrived in Beijing on March 9. He and his colleagues went to Yu's ward and talked with her to learn about her case history and the people with whom she had come into contact since her symptoms began to show.

Following the initial rules of epidemiological study, Zhao and his colleagues identified some 30 people who had come into "close contact" with Yu. They quietly established a network to check on these people. It was later confirmed that four of these people had been infected by SARS and they were sent to hospital for treatment.

They found that Yu could be directly linked with about 20 SARS cases in Shanxi Province, including five medical staff at Shanxi Province People's Hospital, where Yu went for treatment.

Second Chain

On March 20, a patient who had visited a hospital in Beijing earlier that month for a mouth ulcer went to Shanxi Province People's Hospital. He also had a slight fever.

The patient, surnamed Yue, is head of a township in Qingxu County, about 40 kilometers from Taiyuan. Many of his subordinates went to visit Yue at the hospital.

Yue was suspected on March 23 as having been infected with SARS and this was confirmed later. One of the visitors, Yue's driver and the driver's wife soon fell ill with SARS.

Chuo Ruiyi and his colleagues at the Qingxu Center for Disease Control got the news from the provincial disease-control center and began their round-the-clock work.

They subsequently identified seven people as having come into close contact with Yue and the infected visitor and placed 128 more people under what they called "medical observation."

Ma Xiuzhen, the chief accountant of the Qingxu center, recalled: "We only talked to those people about a 'spring respiratory disease' because information on the disease was very limited. Those people couldn't understand why they should be hospitalized."

When Chuo and his staff from Qingxu were tracing Yue's contacts, Zhao Baoxin and his colleagues at the Taiyuan CDC were working on another case that turned out to be the third link of infection in Shanxi.

By April 26, the targets of "epidemic investigations" in Qingxu County numbered some 1,300 people, according to Chuo Ruiyi. Chuo and his colleagues had to work every day until 2 o'clock in the morning.

But Chuo admitted that it had become very difficult to unravel the links.

Hu Shuli, Li Qiyan and Lou Yi from Caijing magazine contributed to this article.

(China Daily May 16, 2003)