Officials from China’s National Health Commission reported an increase in respiratory illness at a Nov. 13 press conference, the WHO said in a Wednesday news release. On Tuesday more reports of pneumonia of an unknown cause in children in northern China were received.

The WHO on Wednesday said it had requested additional information from the Eastern superpower, including patient lab results and data on the circulation of known respiratory pathogens like flu, COVID-19, and RSV. It also requested details on how such circulation is affecting health care system capacity.

Children’s hospitals in Beijing, Liaoning, and elsewhere in the country are “overwhelmed with sick children,” with schools “on the verge of suspension,” according to a Tuesday bulletin from the International Society for Infectious Diseases, citing local news reports.

‘Concerning,’ but likely not new, experts say

Experts Fortune spoke to late Wednesday said they found the cases concerning and called on China to be forthcoming with its data.

But the limited information provided so far doesn’t point to the development of a new pathogen, they said.

The first suspicion of Dr. Michael Osterholm—director of the University of Minnesota’s Center for Infectious Disease Research and Policy (CIDRAP)—is “a series of already recognized respiratory pathogens,” due to the fact that illness are reportedly occurring in children.

“If it was a new pathogen, it would be showing up in kids and adults equally, ” he told Fortune.

Dr. Amesh Adalja, an infectious disease specialist and senior scholar at the Johns Hopkins Center for Health Security, agreed, telling Fortune that while information is sparse, “it would not be surprising if it’s a constellation of various respiratory pathogens together causing clusters of illness.”

Some reporting has pointed to mycoplasma pneumoniae, an atypical bacteria that can cause lung infection, as a potential cause, he said. It’s commonly known as “walking pneumonia.”

“It will be important to gain insights into what pathogens are being tested for and what results are available,” he added, calling the “historical lack of transparency that is emblematic of China’s authoritarian government” problematic, and likely to result in a delay of information.

Dr. Stuart Ray, vice chair of medicine for data integrity and analytics at Johns Hopkins’ Department of Medicine, called the infections “concerning” and said “rapid response and evaluating is the right path.”

“But it’s not so out of the ordinary that we should presume this is due to a new pathogen,” he told Fortune. “We can hope that authorities there will evaluate rapidly, take appropriate steps for mitigating spread, and be forthcoming about the nature and severity of the epidemic.”

Dr. Eric Topol, a professor of molecular medicine at Scripps Research in La Jolla, Calif., and founder and director of the Scripps Research Translational Institute, said it was “impossible to make a comment based on such limited information.”

But in a text to Fortune, he called for “careful watch on this and full transparency from the folks in China.”

WHO calls for masking ‘as appropriate’ in China

While the WHO seeks additional information, it called on Chinese citizens Wednesday to obtain recommended vaccinations, maintain distance from those who are ill, stay home when sick, wear masks “as appropriate,” ensure good ventilation, wash their hands, and obtain medical care as needed.

Northern China has reported an increase in flu-like illness over the past month, when compared to levels seen during the past three years, according to the WHO.

If mycoplasma pneumoniae is to blame, common antibiotics like azithromycin and doxycycline should be effective, Raj Rajnarayanan, assistant dean of research and associate professor at the New York Institute of Technology campus in Jonesboro, Ark., told Fortune.

Health officials should look to pin-down the culprit quickly, ruling out flu—hopefully not a highly pathogenic avian flu—RSV, adenovirus, and COVID, he added.

Jay Weiland, a leading COVID modeler, said there is no reason to suspect a new COVID variant is responsible for the epidemic.

“It’s unclear at this point if this is a collection of simultaneous peaks of a typical seasonal illness or a new infectious illness,” he told Fortune.

Earlier this month Chinese officials attributed the rise in mysterious pneumonia cases to the circulation of viruses like flu and RSV, as well as the lifting of COVID-19 restrictions. But such restrictions terminated a year ago, making the milestone an unlikely culprit.

Still, Osterholm said, it’s possible that China sees a surge in viruses this respiratory season due to the immunity debt theory many blamed for last year’s “tripledemic” of respiratory pathogens in the U.S. and other Western countries.

The so called “immunity debt,” some experts say, is the price society must pay for adhering to pandemic precautions and, thus, receiving less exposure to other viruses. It results, they say, in a delayed surge in non-pandemic viruses after mitigation efforts are abandoned.

Osterholm disagrees with the concept as it applies to last year’s respiratory season in the U.S. While it got off to an early start, respiratory viruses didn’t cause more deaths than usual, he points out—nor were “lock downs” in the United States true lock downs. In U.S. states, stay-at-home orders lasted only months—not years, like in China—and didn’t apply to “essential workers.” In Minnesota, that meant 82% of the state’s population was exempt, Osterholm said, adding, “that’s not a lock down.”

China, with its much more stringent regulations, truly locked down, he said. And, thus, it might expect to see a resurgence of other viruses this year, in “immunity debt” fashion.

“I think you’re going to see more of this happening around the world with seasonal respiratory viruses,” he said. The next respiratory season or two may be atypical, he added—perhaps due to “immunity debt,” or perhaps due to viral interference—in which competitive viruses like COVID “cancel out” other viruses for a period of time. Such a phenomenon appears to have happened during the H1N1 bird flu pandemic of 2009, during which other strains of flu and RSV “disappeared” for a time.

An alternate theory that could explain post-pandemic surges in other viruses, according to some experts like Ryan Gregory, a biology professor at the University of Guelph in Ontario, Canada: that COVID suppresses the immune system—at least temporarily, and at least in some—making them more susceptible to other infections. Another: that being infected with both COVID and another pathogen at the same time makes what would otherwise be mild illness more severe.

“Assuming immunity debt as the explanation may miss something important and concerning,” he told Fortune. “The idea that the way out of situations like this is for people—especially children—to get infected is problematic. Not everything is COVID, but not nothing is COVID either, and we should not be content to have this be our new normal.”

As for what exactly is going on in China, two pieces of data are needed, according to Osterholm: “accurate epidemiological data describing who is sick, and lab-based data.”

“Then we can answer in short order what’s going on.”

While the epidemic must be taken “most seriously by those closest to the problem,” in China, “we also need to realize how small our world is now,” Ray added. “We’re all neighbors.”