Chinese researchers are racing to uncover the cause of a mysterious respiratory illness that has infected almost 60 people in central China. Authorities have ruled out the highly infectious severe acute respiratory syndrome (SARS), which killed hundreds of people there in 2002–03. But missteps during that disaster may explain why scientists are being cautious about releasing information on the latest outbreak, say researchers outside the country.

As of 5 January, 59 people in the east-central city of Wuhan have been infected with the mystery virus, with 7 in a critical condition, according to the Wuhan Municipal Health Commission. The health commission says that it has also ruled out Middle East respiratory syndrome, avian influenza and other influenza viruses as the cause, based on advice from infectious disease experts in China, including researchers at the Wuhan Institute of Virology, which houses one of the country’s premier biosafety laboratories . Symptoms of the illness include fever and difficulty breathing, which are common to several respiratory diseases, according to the World Health Organization, which released an emergency preparedness report on the same day.

The municipal health commission says that it is trying to isolate the virus from patients to identify the pathogen and its potential source. Many of the people who have become ill work in a seafood and live animal market in Wuhan, which was shut down on 1 January. The health commission says there’s no evidence that the infection is being passed between people, suggesting it is not transmitted easily in humans.

The infection is probably an emerging zoonotic virus — infections that spread from animals to humans, says Linfa Wang, a virologist at Duke–National University of Singapore. Such infections often first appear in colder months when animal pathogens can survive and spread to people, he says.

But the scientists studying the pathogen are unlikely to release their findings until they’re confident with the result, says Wang, who works on emerging viruses with Chinese research groups but hasn’t been involved in this outbreak. He thinks many scientists will remember how, in the early months of the SARS outbreak before it had been identified as a coronavirus, some Chinese scientists announced prematurely that cases of atypical pneumonia were caused by chlamydia, delaying measures to contain the outbreak. Wang says that this is likely why scientists are being cautious about releasing information about this latest illness.

Christian Drosten, a virologist at the Charité – University Medicine Berlin, suspects that the researchers have already identified the virus, but are waiting for government permission to announce their results, or have them confirmed in other laboratories. “There are good reasons to not communicate results early,” he says.

Drosten thinks the pathogen might be a coronavirus that is related to SARS, which also points to an animal source.

Although the virus currently shows no signs of being transmitted between people, it could develop that capacity over time and lead to and epidemic, says Seungtaek Kim, a virologist at the Institut Pasteur Korea in Seongnam, South Korea. “Since we do not know the identity of this pathogen, it would be safe to treat this outbreak as SARS to prevent further spread,” he says.

The health commission has isolated the infected people and is monitoring those in close contact with them, as well as looking for related cases that might have been missed.

Authorities in nearby Hong Kong and South Korea have also started screening travelers who have recently been to Wuhan. In Hong Kong, 30 people who had recently visited Wuhan presented with respiratory illness. Authorities say that most had influenza or other infections, but ten of these cases are still unidentified.

The current spate of infections seem to be less serious than the 2002–03 SARS outbreak, says Wang, because there have been no deaths, or reports of medical staff becoming ill — usually a sign that an infection is spreading among people and might cause an outbreak.