In January, a person infected with the dangerous new Wuhan coronavirus used public transportation to crisscross the eastern Chinese city of Nanjing, potentially exposing those along the way to the highly contagious pathogen.
Using the country's pervasive digital-surveillance apparatus, authorities were able to track--down to the minute--the sick person's exact journey through the city's subway system.
Officials then published those and other details of the person's movements on social media and warned residents to get themselves checked if they had been in the vicinity at the time.
"In this day and age, you can trace everyone's movements with big data," Li Lanjuan, an adviser to the National Health Commission, said in an interview with state television.
China's government has launched an unprecedented effort to track the fast-spreading virus, which has infected more than 20,000 people and killed at least 425 around the world.
Much of the work is being done by armies of neighborhood monitors and managers of residential complexes, tasked with checking on people believed to have recently traveled to Hubei province, the epicenter of the outbreak, and reporting their findings to authorities.
But the human virus trackers have help. Authorities are sifting through information from phone companies, railroads and airlines as they tackle the country's biggest public-health crisis since an outbreak of severe acute respiratory syndrome, or SARS, almost two decades ago.
China has built a formidable digital-surveillance system in recent years, linking facial recognition, security cameras and social-media monitoring with regular human surveillance. The aim is to keep tabs on its 1.4 billion citizens, chiefly to identify and prevent threats to social order and Communist Party control.
The country's state-run media has celebrated the application of the authorities' big-data tracking abilities in the campaign to control the disease outbreak, touting it as an example of the social benefits of technology.
Shizhu county in Chongqing, in southwestern China, said it used unspecified data to track more than 5,500 people arriving for the Lunar New Year holiday from Hubei province. Roughly half were placed under home quarantine, the Communist Party-run Chongqing Daily newspaper reported.
In Zhejiang province, not far from Nanjing, authorities dealing with one patient who denied having contact with anyone in Wuhan were able to use data analysis to show he had interacted with three others from the city, China Central Television reported last week.
Chinese officials not only have access to more data than most other governments, but they can also tap it more quickly thanks to close relationships with the mostly state-run companies that oversee the country's transportation and telecommunications networks.
That has allowed China to partially automate the process of tracking the movements of infected individuals--a process known as contact tracing--which most countries still do using face-to-face interviews.
In China, "there is a much greater capacity to access and analyze big data, so the potential scale of contact tracing that could be undertaken is much larger," said Nic Geard, an engineering professor who studies data modeling of diseases at the University of Melbourne.
Mr. Geard, however, cautioned about risks to individual privacy. Even if data is redacted, "there can be enough personally identifying information such that you can reconstruct the person's identity," he said.
An online leak of personal information, including home addresses, belonging to hundreds of people who had traveled home from Wuhan illustrates the dangers. Several in the group, many of them university students, reported being bombarded with harassing phone calls and messages on social media.
In a notice published last week, China's Ministry of Transport warned taxi and ride-hailing companies to take greater care in protecting user privacy when transferring data to the National Health Commission.
Some experts argue that privacy matters less in an emergency. "In the event of a public-safety threat, it is OK to sacrifice some privacy for the benefit of society," said Zhu Wei, associate professor at China University of Political Science and Law and adviser to the government on legal issues in cyberspace.
Chinese citizens are accustomed to handing over personally identifying information to the government and organizations like the country's state-run railway.
Huang Xin, an executive with China State Railway Group Co., said at a press conference in Beijing last week that it had set up a task force to help response teams track passenger traffic and identify people who have shared cars with virus patients. The company had provided such data more than 200 times in the 30 days since the start of the outbreak, he said.
Transportation authorities said last week that, in addition to entering national ID numbers, all railway passengers now also need to provide mobile-phone numbers to purchase tickets.
China's tech giants have also sought to adopt big data to fight the outbreak. Chinese search-engine and mapping provider Baidu Inc. produced an online map that allows users to track the final destination of outbound car trips from Wuhan in the days leading to the city's shutdown. Sogou, another Beijing-based search engine, runs an app allowing travelers to search retrospectively if they were on the same flights or trains as infected victims.
Smaller startups introduced maps for Chinese netizens to track the number of virus infections in a neighborhood in real time.
China's three big state-run telecom operators contribute to the tracking by using network signals to monitor the location of mobile phones, said Huang Huang, a data scientist at Peking University's School of Government. Mobile-phone numbers in China are tied to national ID numbers, meaning it is easy for carriers to identify the owner of almost any device, Mr. Huang said.
One of those carriers, China Unicom, created a big-data team of more than 100 people to help the government's outbreak response team track population movements, the official Xinhua News Agency reported.
None of the three telecom carriers responded to requests for comment. China's Ministry of Industry and Information Technology, which regulates data-sharing, also didn't respond to a request for comment. China has been on an extended Lunar New Year holiday break because of the coronavirus.
West Africa has tried to use information from telecom carriers to fight the spread of Ebola in the past, but those efforts were hindered by difficulty identifying the owners of mobile-phone numbers, patchy mobile network coverage and pushback from privacy advocates, said Susan Erikson, a professor of health sciences at Simon Fraser University in Vancouver, Canada.
Few of those problems come into play in China, where roughly 850 million mobile internet users are served by 4G networks that cover almost the entire country. Concern over data privacy, though growing in major cities, is small to nonexistent in most places.
Ms. Erikson, who spent time in Sierra Leone during the Ebola crisis, said the idea of using big data often sounds more impressive than it is.
"In reality, there is too little return on the massive investment required to translate telecom info into public-health action," she added. "The opportunity costs in an emergency are huge. Paying attention to low-return activity during a crisis means not paying attention to other more urgent public-health needs."
Yang Jie contributed to this article.