Stoking fears that a novel virus may have begun to infect people, health authorities in the central Chinese city of Wuhan late on Friday local time announced they have documented 44 unusual cases of pneumonia—a sharp jump from their initial report of 27 cases on 31 December 2019. Although the city is being praised for quickly sharing information, infectious disease specialists around the world are eager to get more details on the mysterious pathogen and the disease it produces in patients. In today’s report, Wuhan officials ruled out influenza, avian flu, and adenovirus; they call the disease a “viral pneumonia of unknown cause.”

Hints of trouble first surfaced publicly on 30 December, when a directive from the Wuhan Municipal Health Commission asking hospitals to report unusual cases of pneumonia was reported by local media. The next day, the commission posted a notice in Chinese on its website stating that a number of local hospitals had reported cases of pneumonia linked to the wholesale Huanan Seafood Market. The commission had turned up 27 cases in the city of 11 million, 690 kilometers west of Shanghai. Seven of the patients were in serious condition, two had recovered and were nearing discharge, and the remaining number were stable. All patients were isolated and their close contacts were under surveillance, the notice stated.

At that time, no human-to-human transmission had been identified. The commission updated that information today, saying 11 of the 44 cases are considered serious. The new patients are also in isolation. An additional 121 close contacts of patients are under surveillance, although the commission has so far ruled out human-to-human transmission.

The city has shut the seafood market, according to local press reports. The quick response and public notification “is obviously progress for China” says Guan Yi of the University of Hong Kong, an expert in emerging viral diseases. China had been criticized for its slow action and a lack of transparency when severe acute respiratory syndrome (SARS), caused by a previously unknown virus, emerged in 2002 in southern China. In response, it beefed up disease surveillance efforts.

Spotting the linked pneumonia cases “shows the benefit of a surveillance system,” Guan says. At any time during winter, he notes, every big city hospital is likely to have dozens of pneumonia patients, making unusual cases hard to spot.

Guan says there are unconfirmed reports of a mother and her son contracting the same illness, a situation that would have stood out and was, perhaps, something that caught the health commission’s attention. Guan, who is not involved in the investigations in Wuhan, says this does not necessarily involve human-to-human transmission; the son reportedly ran a stall at the implicated seafood market and his mother frequently visited him there. “They could have been exposed [to the same pathogen] simultaneously,” he says.

Local media have reported that some market vendors also sold live animals, including birds and snakes, and the organs of rabbits and other wildlife, raising concerns about a zoonotic pathogen jumping to humans. Rumors had circulated online claiming that this could be the return of SARS, a zoonotic disease that spread to 37 countries, infecting more than 8000 people and causing 774 deaths before it died out. The disease was traced to a novel coronavirus that had mutated as it jumped from bats to civets sold in live animal markets, where a few people picked it up and transmitted it to others. There was no vaccine and no effective treatment for SARS, aside from supportive steps such as mechanical breathing support and the administration of steroids to reduce lung inflammation.

Guan, who played a key role in unraveling the source of the SARS virus, says the recent pneumonia patients are apparently responding to standard supportive treatments, so, “I don’t think this is SARS.” Given that suspicions center on the fish market, authorities should “work to dig out the zoonotic source,” Guan says.

Local media reported that Wuhan authorities arrested a number of individuals for allegedly spreading unverified information online, “causing an adverse social impact.”

Aside from the notice posted by the Wuhan health commission, little—if any—additional information has been released. And the dearth of details is frustrating scientists both in China and around the world. An official of the Beijing office of the World Health Organization told the South China Morning Post that “WHO is closely monitoring this event and will share more details as we have them.” The WHO office did not immediately respond to a Science request for an update.

“We do not know if influenza tests were performed, or if tests for the SARS [coronavirus] are underway,” Marjorie Pollack, a moderator of the Program for Monitoring Emerging Diseases, a widely followed internet service tracking infectious disease outbreaks worldwide, wrote online. “More information on this outbreak including demographics of cases, possible known common contacts, and a clinical description of the illness would be greatly appreciated.”