The cause of mysterious pneumonia cases in the Chinese city of Wuhan remains unknown, health authorities in the city said Sunday, as the number of infected people rose to 59 from 44 on Friday.

Seven of the sick are listed as critically ill, down from 11 on Friday. The number of close contacts of cases under medical observation has risen to 163.

Sunday’s statement, the third from the Wuhan Municipal Health Commission about the incident, is the first to give information about when people became infected. The first person known to have become ill began to show symptoms on Dec. 12 and the last date of symptom onset among the sick was Dec. 29, the statement said.

Preliminary investigations show no clear evidence of person-to-person spread of the infection, and no cases among health workers, the Wuhan authorities said. The latter fact is especially important.

Infections among medical staff are common at the start of infectious diseases outbreaks, when health workers don’t know they need to take special precautions to protect themselves.

“I would say that … given the dates that they have mentioned there and the fact that no health care workers have been infected would suggest it’s not transmitting efficiently in humans,” said Ralph Baric, who studies emerging viruses at the University of North Carolina.

“So it sounds to me like it’s something that’s jumped from animals to humans, and it’s in early stages. And right now it’s virus evolution versus public health control measures to try to prevent spread. That’s what’s going on,” he said.

The Wuhan statement listed some pathogens Chinese scientists feel confident are not responsible for this outbreak: regular influenza, avian influenza, adenoviruses (which can cause severe respiratory infections), Middle East Respiratory Syndrome (MERS) or SARS.

Rumors have swirled on social media platforms that the outbreak might be a return of SARS, a disease that exploded from China to touch off outbreaks in multiple Southeast Asian countries and in Toronto, Canada in early 2003. The outbreak was successfully halted in the summer of 2003 and with the exception of a few cases the next year, SARS hasn’t been reported since.

Symptoms of the current outbreak are mainly fever, though a few patients are having difficulty breathing, the World Health Organization said Sunday in its first update on this outbreak. The WHO said chest X-rays show invasive lesions in both lungs.

News of the outbreak emerged on Dec. 31; that is when China alerted the WHO to the findings, the global health agency reported. The following day Chinese authorities closed and decontaminated the South China Seafood Wholesale Market, where some of the infected worked. Despite its name, the market also sells wild animals for meat.

The slaughtering of wild animals and preparation of their meat can lead to the transmission of disease. The SARS virus came to humans from civet cats, eaten as a delicacy in southern China. Ebola outbreaks — there is no belief this is one — are often linked to bush meat preparation.

If a new virus is causing the pneumonia cases, it likely comes from birds or animals, not seafood, said Michael Osterholm, director of the Center for Infectious Diseases Research and Policy.

He praised the Chinese authorities for the information they have make public so far.

“This much information this early into an outbreak is a very positive thing,” Osterholm said. “I feel confident at this point that the appropriate public health measures are being taken to both investigate the outbreak and to contain it as much as possible…. And I think hopefully over the next several days it will become much clearer what the [causative] agent is.”

While information is emerging, infectious diseases experts watching this outbreak would like more. Baric, for instance, would like to know how old the cases are. Some pathogens cause more severe illness in the elderly or the very young.

“If they are all 65 year olds, then that starts screaming ‘coronavirus’ to me,” he said.

Osterholm said it is critical to establish — or share, if one has been established — a case definition. That’s the tool public health officials use to rule in and out possible cases in an outbreak.

When an outbreak triggered by an unknown cause, there is no test. So figuring out a way to differentiate people who are probably part of the outbreak from people who are sick with other viruses is critical.

Until then, public health has to throw out a big net, pulling in people who are sick with similar symptoms and who were in Wuhan, but who may in reality have nothing to do with this outbreak.

A report public health authorities in Hong Kong released Sunday shows the problem. They have to date identified 16 suspect cases, all with recent travel to Wuhan. So far 10 of them have tested positive for a mix of known respiratory illnesses — flu, parainfluenza, rhinovirus (the leading cause of the common cold), RSV or respiratory syncytial virus and others.

Seven of the 16 illnesses were caused by flu, which suggests Wuhan may be in the midst of a flu epidemic, said Marion Koopmans, director of department of virology at Erasmus Medical Center in Rotterdam, the Netherland. “[I’m] not sure what the testing in Wuhan looks like, but this could still be a pseudo-outbreak if flu activity is high,” she said in an email.

But until more is known, potential cases — both in Wuhan and in places further afield — will have to be isolated and monitored. On Sunday, officials in Singapore reported they had identified a suspect case, a 3-year-old Chinese girl who had recently traveled to Wuhan.

“The good news is they’re aware,” Baric said of the way Chinese authorities have quickly jumped on these suspicious pneumonia cases. “The better news would be if they knew what it was and could provide a diagnostic test … so that every single person who goes from Wuhan to Hong Kong or Singapore who has a fever isn’t suddenly quarantined for [what turns out to be] an RSV infection.”

Republished with permission from STAT. This article originally appeared on January 5 2019