The strain—first identified in domestic waterfowl in China in 1996—is behind nearly 58 million U.S. bird deaths in the past year. These deaths have occurred both directly due to the virus and indirectly, when flocks are culled to curb further exposure, according to the U.S. Centers for Disease Control and Prevention. The virus has a near 100% fatality rate among birds, killing most infected within 48 hours.

This season’s bird flu outbreak is the worst in U.S. history, having surpassed a 2015 outbreak the CDC once called “arguably the most significant animal health event in U.S. history.” That year, nearly 51 million birds died nationally due to H5N1 and related avian flu viruses. This season’s outbreak is also the worst in U.K. history, with farmers in England ordered to keep their birds indoors as of Nov. 7 in a poultry “lock down” of sorts.

The so-called “R naught” value—or the number of people infected by a single infected person, on average—for COVID initially ranged from 1.5 to 7, and now sits upwards of 12. 

The R naught value of H5N1 among birds: “around 100,” according to Chowdhury.

It’s an environmental crisis already impacting humans, sending the price of poultry skyrocketing and making eggs harder to come by. U.S. egg retail costs have doubled over the past year, and oven-ready chicken prices are up by a quarter or more in the U.K., Bloomberg recently reported.

“The world is facing an unprecedented pandemic of avian flu among caged and wild bird populations,” Rajiv Chowdhury, senior epidemiologist and professor of global health at Florida International University, tells Fortune.  

What’s more, the poultry industry likely cannot “sustain itself in countries like the U.S. if we continue to see annual surges,” Dr. Michael Osterholm, director of the University of Minnesota’s Center for Infectious Disease Research and Policy (CIDRAP), adds.

Economic and food-supply issues aside, the strain has sickened people, too, albeit in small numbers so far. But the trend of minimal transmission among humans may not hold, experts warn. Last week the WHO reported that a previously healthy Ecuadorian girl had been hospitalized with the virus. Just how she contracted the virus is uncertain and under investigation, but poultry her family had recently purchased died without apparent cause. 

Oserholm and Chowdhury are concerned about additional spread from birds to humans, and about possible transmission from humans who contract the bird flu to other humans—especially as the virus traverses the globe and makes the leap to more mammals, due to mutations. 

Last week the first grizzlies to be documented with the highly pathogenic avian flu were euthanized in Montana after they were found partially blind and disoriented, with other neurologic issues. Foxes, dolphins, opossums, skunks, seals, other types of bears, and a bottlenose dolphin are among other species that have been infected since last year, according to the U.S. Department of Agriculture.

The likelihood of human-to-human transmission of H5N1 is “very low,” Chowdhury says. But if it were to occur in a sustained manner, it could rock the globe in a way not seen since the 1918 Spanish Flu.

If H5N1 indeed makes a sustained crossover to humans, “the potential impact could be significant,” he says, signifying the start of a “new global influenza pandemic.”

‘Unusual’ among humans, but highly fatal

The virus’s Ecuadorian victim—the country’s first, as well as the first in the Latin America/Caribbean region—was admitted to a hospital for symptoms of nausea, vomiting, and constipation on Dec. 30, where she was presumptively treated for meningitis, according to a Jan. 18 WHO disease outbreak update.On Jan. 3, she was transported to a pediatric hospital in critical condition after suffering septic shock and being diagnosed with pneumonia. She tested positive for the H5N1 strain on Jan. 7 and remained hospitalized, under sedation and on a ventilator as of Jan. 17, the international health organization stated.

The previously healthy girl becomes the seventh individual the virus has sickened since 2020, according to the World Health Organization. While H5N1 is considered highly infectious, that’s only among birds. It’s typically difficult for the virus to make the leap to humans, and transmission from human to human is “unusual,” the international health organization says.

When the virus does make the leap, however, it’s highly fatal, with a mortality rate of greater than 50% among humans, according to the CDC.

H5N1 outbreaks have come and gone throughout a quarter century, without sustained transmission in humans, Osterholm points out. Hundreds of human cases were identified in Egypt earlier this century, though there was no sustained human-to-human transmission. The virus’ track record gives reason to hope that transmission to humans, and among them, will remain the exception rather than the rule.

Chowdhury says the 1918 flu pandemic, however, serves as a cautionary tale. Like the H5N1 flu, the Spanish Flu is thought to have avian origins. Both viruses contain genes that allow them to replicate efficiently in human bronchial cells, according to a 2006 reportfrom the Washington, D.C.-based Population Reference Bureau. 

In the case of H5N1 among humans, such inflammation can lead to lung cells becoming “intensely inflamed”—much more so than would be seen in a usual flu. A similar effect was noted in Spanish Flu victims, autopsies of which revealed “lungs choked with debris from the excessive inflammation,” resulting in drowning, the report noted.

While seasonal flu symptoms include fever, cough, sore throat, runny nose, aches, and fatigue, according to the CDC, symptoms of H5N1 in humans are typically much more severe. They include an often high fever, weakness, cough, sore throat, and muscle aches, abdominal pain, chest pain, and diarrhea, according to the WHO. These symptoms can quickly give way to difficulty breathing, pneumonia, and/or Acute Respiratory Distress Syndrome, which is often deadly, as well as neurologic effects like seizures.

When it comes to planning for the next pandemic, influenza has always been considered a likely culprit, Dr. Georges Benjamin, executive director of the American Public Health Association, tells Fortune.

While the Ecuadorian girl and her disturbing bout with H5N1 “may very well be a one-off” case, “we could easily have an H5N1 or another pandemic of influenza,” he says.

“It should scare you.”

While there’s currently no reason to suspect sustained H5N1 transmission among humans, the outlook may not be so hopeful for the U.S. poultry industry, which is in “deep trouble right now,” Osterholm tells Fortune.

“They’ve got to change biosecurity procedures,” he says, adding that the virus can’t be kept away from flocks simply by “putting a screen up” because it’s transmitted in an airborne manner.

“These barns are going to require a great deal more air-handling considerations,” he adds. “H5N1 has fundamentally rewritten avian influenza.”