But “white lung” pneumonia isn’t a thing, experts tell Fortune, and certainly not a medical term. It’s one way to describe the X-rays of some who have pneumonia, which has a number of potential causes. And there is no sign of an unusual upward surge, with rates of pneumonia diagnoses in children sitting around average, according to national health data available to providers. 

Pneumonia is an infection of one or both lungs. It causes air sacs to fill up with fluid or pus. And “by definition, [it] causes whiteness in the appearance of the lungs on a chest X-ray,” Dr. Amesh Adalja, an infectious disease specialist and senior scholar at the Johns Hopkins Center for Health Security, tells Fortune.

It’s the most common cause of both pediatric and adult hospital admission in the U.S., with childbirth being the exception for adults. “Every year around the winter season in the United States, pneumonia characteristically rises,” Adalja adds. “There is no evidence that there is any increase prior to seasonal averages currently.”

Pneumonia cases in China unrelated, experts say

Likely behind the trending but misleading “white lung” phrase: a fear that cases of pneumonia of an “unknown origin” among China’s children have spread to European countries and the U.S. But such cases are being fueled by known pathogens like RSV, flu, and mycoplasma pneumonia, experts say. What’s more, the type of X-ray reportedly seen in connection with many Chinese cases has a different appearance: not white, but with “ground-glass opacities,” which can signal more serious illness. 

“The first thing everybody feared was they had ‘seen this movie before,’” Dr. William Schaffner, professor of infectious diseases at Vanderbilt University in Nashville, Tenn., tells Fortune regarding the World Health Organization’s Nov. 23 announcement about the mysterious cases of pneumonia.

But Chinese authorities provided further information to the WHO and cleared up the matter, Schaffner says, adding that the world may see another winter of increased infections after the lifting of COVID precautions. And there’s certainly more attention paid to infectious disease these days, adds Dr. Steve Morse, professor of epidemiology at Columbia University.

“Everyone’s a little bit on edge because of the pandemic we’ve just had,” Morse tells Fortune. “I think there’s a lesson there, that we may be a little more aware of these things—including things that may not actually be unusual—after the pandemic.”

The concern this season is only for the typical illnesses, experts say—flu, RSV, COVID, and influenza-like illnesses. None are new, and all can lead to pneumonia, as they’ve always been capable of doing.

And while the name “mycoplasma pneumonia” might be new to most, it’s far from a new pathogen. Caused by bacteria, it’s a “common cause of respiratory infection worldwide, with cases appearing year-round in different regions and local outbreaks occurring every few years,” Rajiv Chowdhury, professor of global health at the Robert Stempel College of Public Health and Social Work at Florida International University in Miami, tells Fortune. 

What’s more, current global levels of mycoplasma pneumonia appear to be hovering below pre-pandemic levels, Chowdhury says, citing research recently published in Lancet Microbe.