In the last two years, more than half a billion birds have died globally. The cause isn’t deforestation or climate change or the destruction of grasslands — all of which are contributing to the precipitous decline of wild birds — but avian influenza, i.e., bird flu.

The majority of these birds were farmed chickens and turkeys; as the virus, known as H5N1, began circulating among poultry flocks in the US, Europe, and elsewhere, farmers started culling a record number of birds to stop the pathogen from spreading.

Yet what’s unusual about this virus is that it’s also been spreading rapidly among wild birds and even mammals, such as mink and sea lions, often causing severe infection or death. This raises a red flag among health officials.

“The increasing number of H5N1 avian influenza detections among mammals — which are biologically closer to humans than birds are — raises concern that the virus might adapt to infect humans more easily,” three United Nations agencies, including the World Health Organization, warned in a recent statement. “These outbreaks pose ongoing risks to humans.”

Bruna Prado/AP

The risk of humans contracting bird flu is still incredibly low, new research shows, and the virus does not have the biological machinery to cause a pandemic (unlike the SARS-CoV-2 virus, which is great at spreading among humans). But the more this pathogen spreads, especially among mammals, the more opportunities it has to evolve traits that make it dangerous. That’s why scientists are taking it so seriously.

What is bird flu?

Avian influenza, or bird flu, is a general term for the disease caused by influenza viruses that primarily infect and spread among poultry and some wild birds. They’re distinct from viruses that cause the flu in humans, though they’re related.

These avian viruses are common. They’ve been circulating for eons among wild waterfowl, such as ducks and geese, without causing them much harm. Mild forms of infection are called low-pathogenic avian influenza, or LPAI, which means they’re typically not deadly.

The type of avian influenza spreading today is different.

Occasionally, an LPAI virus can jump from wild birds to poultry farms. As the virus replicates in densely packed warehouses of farmed birds, it can quickly evolve and pick up adaptations that make it highly deadly to poultry. At that point, it gets dubbed a highly pathogenic avian influenza virus, or HPAI virus. That’s what’s circulating today — an HPAI virus.

Still, these more deadly forms of avian influenza viruses are not new. They’ve caused outbreaks on poultry farms many times in the past that have killed thousands to millions of farm birds. What makes this particular form of avian influenza virus so unusual is that it easily spreads and causes severe disease among wild birds and — importantly — some mammals.

Why is it in the news right now?

The particular type of avian influenza spreading today descended from a virus that caused an outbreak on a goose farm in Guangdong, China, in 1996. That virus — one of a type of virus known as H5N1 — was highly pathogenic and killed more than 40 percent of the farm birds it infected.

Since then, H5N1 viruses of Guangdong descent have caused sporadic outbreaks among poultry and some wild bird populations, largely in the Eastern Hemisphere. But around late 2021, the HPAI virus started going global, arriving in North America and then spreading south.

Bird flu first became big news in the US when it started spreading among poultry farms, forcing farmers to cull millions of egg-laying hens. That threatened the livelihood of farmers and sent egg prices soaring in late 2022 and early this year.

Then more and more reports of the virus spreading among wild birds and both farmed and wild mammals emerged, raising concern among health officials and independent scientists. That made it clear that this virus is different than past forms of avian influenza — and potentially a greater risk to humans.

Last summer, thousands of dead seabirds began washing up on the shores of eastern Canada. That fall, meanwhile, avian flu infected large numbers of mink at a fur farm in northwestern Spain. On the farm, the virus picked up a genetic mutation that could make it better at replicating within mammalian cells, raising concerns that it could eventually jump over to humans.

This form of HPAI virus has now spread to every continent other than Australia and Antarctica. Scientists now say it’s causing a “panzootic,” meaning a pandemic among animals, and health officials are racing to get it under control.

What kinds of animals are getting infected?

What makes the virus particularly troubling is that it infects an incredibly wide array of both farmed and wild animals. The list includes more than 150 species of wild birds — from terns to owls to vultures — including those that are already threatened with extinction for other reasons.

Since March, for example, more than 20 endangered California condors in the Southwest have died, and most of them tested positive for avian flu. The largest bird in North America, with a wingspan that can reach more than 9 feet, the California condor almost went extinct in the 1980s due to lead poisoning and other factors. A successful captive breeding program revived the population, which now stands at roughly 550 worldwide.

Richard Vogel/AP

These sorts of die-offs are troubling, ecologists say.

“Large die-offs can impact populations of these species for decades and may contribute to species collapse and further ecosystem damage, particularly given the critical declines seen in North American bird biodiversity over the last half century,” scientists wrote in the April study, which tallied reported and suspected cases of H5N1 in North American birds.

Then there is the growing list of infected mammals. Outbreaks among mink and marine mammals have garnered the most attention; earlier this year, officials reported that bird flu killed almost 3,500 seal lions in Peru and thousands more in Chile. Officials have also detected the virus in dozens of other wild animal species including tigers, bears, otters, raccoons, coyotes, and dolphins. Some pet dogs and cats have also been infected (more on that below).

How many birds have died?

Globally, the current outbreak has killed — or forced farmers to cull — more than half a billion poultry worldwide, according to a preprint (i.e., not yet peer-reviewed) paper published in May. In the US, that number is roughly 59 million as of July 12.

The toll the virus has taken on wild birds is more difficult to pinpoint.

In the US, suspected or confirmed cases of H5N1 in wild birds are in the tens of thousands, according to a study published in April. Reporting by the Guardian revealed that the flu has killed more than 50,000 birds in the UK. In eastern Canada alone, roughly 40,000 birds have been reported as sick or dead, likely linked to the flu, as I previously reported.

Most of these numbers are likely underestimates. Government agencies don’t have the resources to test every dead bird. Plus, many individuals die out at sea, or in rural areas that lack surveillance. In reality, the total number of dead birds is likely in the millions globally, according to the preprint.

Seth Wenig/AP

Is bird flu still inflating egg prices?

As avian flu swept through poultry farms last year, farmers killed millions of birds. Fewer birds meant fewer eggs, and fewer eggs sent egg prices soaring. In December of 2021, the US had roughly 328 million egg-laying hens; a year later, that number had fallen to about 308 million.

While avian flu is still spreading among wild animals globally, health officials and farmers have largely managed to curtail infection among poultry in recent months, at least in the US. And that’s caused egg prices to drop. The wholesale cost of a dozen eggs in the US is now just over a dollar, compared to around $5 in parts of the country earlier this year.

Can humans get bird flu?

Yes, we can, though infection is highly uncommon.

Prior to the current outbreak, several hundred people had been infected by H5N1 viruses (similar to the one spreading today) and more than half of them died, according to the Centers for Disease Control and Prevention.

But that should not alarm you. Even though avian flu is currently somewhat widespread among birds and some mammals, it rarely spreads to humans, according to the WHO. Since late 2021, when the virus was going global, health officials have only reported eight cases of H5N1 infection in humans. At least one of them, an 11-year-old girl in Cambodia, died.

Nearly all cases of human infection involve people interacting with loads of sick birds on poultry farms. That indicates that the virus is likely only capable of infecting a human if they are exposed to an enormous viral load.

What are the symptoms of bird flu?

Sick birds can shed the virus through their saliva, mucus, and droppings. Humans risk infection when these infectious substances get into their eyes, nose, or mouth. (Fortunately, most people naturally want to avoid bird poop in their mouths.)

What’s a bit confusing about bird flu symptoms is that they can vary a lot — and they can overlap with those of other respiratory diseases including Covid-19. Symptoms can be nonexistent or subtle, such as eye redness or a mild flu, or show up as a high fever (above 100 degrees Fahrenheit) and pneumonia. Coughs, sore throats, congestion, body aches, headaches, fatigue, difficulty breathing, and diarrhea have also been linked to bird flu, according to the CDC.

If you’ve been exposed to birds infected with avian influenza, you should monitor yourself for any signs of illness for 10 days, per the CDC. And if you have any of them, err on the side of caution and contact your local health department. (Here’s a directory.)

Erin Hooley/AP

Could bird flu become a human pandemic?

The avian influenza virus circulating today is almost certainly incapable of spreading dangerous infections to people around the planet. Simply put, it does not have the proper biological machinery to easily invade — let alone circulate among — humans. If you want to go a layer deeper, here’s how my coworker Dr. Keren Landman (a physician) and I explained it earlier this year:

To infiltrate a host, viruses first have to bind to certain receptors on the surface of their cells. The virus that’s currently spreading, H5N1, does this using a specific kind of protein known as hemagglutinin 5, or H5. You can think of H5 as a key and receptors as the locks.

Following this metaphor, H5 can unlock certain receptors found in cells that line the respiratory and digestive tracts of birds. By invading those cells and replicating, the virus can damage these vital systems, making it difficult for the birds to breathe and easy for them to spread the virus among themselves (through breath and feces).

Humans have some similar, avian-type receptors in our respiratory systems, too. But for reasons scientists don’t fully understand, they don’t make us as vulnerable to avian flu as birds are. Critically, we also have a higher number of different, non-avian-type receptor that bird flu viruses don’t like to bind to quite as much. The abundance of those non-avian receptors in our noses seems to protect us from being easily infected by viruses like H5N1.

The bottom line: In its current form, the virus doesn’t easily bind to cells in our airways, so it’s harder for the virus to infect us.

But even if HPAI could easily infect humans, and replicate within our cells, it wouldn’t necessarily become a public health emergency. Typically, for a virus to cause a pandemic, it must be human-to-human transmissible — particularly through the air, like how SARS-CoV-2 spreads — and, at this time, there’s no indication that this virus has that ability.

“With the information available so far, the virus does not appear to be able to transmit from one person to another easily,” said Dr. Sylvie Briand, WHO’s director of epidemic and pandemic preparedness and prevention, in a recent press release.

So why are health officials concerned?

Viruses evolve quickly, and unpredictably. Not only do they mutate, but they can also swap entire portions of their genomes with other viruses — creating new frankenviruses — if two or more of them infect the same host. Under the right circumstances, this evolution could give the virus the tools to replicate more easily in mammals, which would make them more threatening to humans.

That’s what makes the outbreak among wild birds today so concerning. The more the virus spreads, the more opportunities it has to evolve dangerous traits.

During most past outbreaks, only poultry farms were badly infected, so farmers and health officials could kill giant flocks of infected farm birds and exercise other biosecurity measures to stem the spread. But now that wild birds are a reservoir for HPAI, containment is much, much harder. Indeed, no matter how much culling farmers do, wild flocks could still pass H5N1 over to domestic populations.

Once wild birds contaminate farms, the risks skyrocket. Viruses evolve much more quickly within animals that are packed tightly together, simply because they have more bodies to grow in. On farms, they’re also known to pick up adaptations that make them better equipped to replicate among mammals, Wendy Puryear, a molecular virologist at Tufts University, told Vox earlier this year. (It’s not clear why).

Another challenge is that when the virus is widespread among wild birds, it has more opportunities to spill over directly into mammals. That’s how all those seals and mink and other mammals likely got sick, Puryear said — they came into contact with infected birds. And as these microbes replicate within their cells, the flu viruses can pick up traits that make them more harmful to humans.

Robert F. Bukaty/AP

This is not just theoretical. In a recent study, Puryear found that some H5N1 viruses infecting seals in New England had genetic changes that have been shown to make them more efficient at replicating within the cells of mammals. Researchers have found similar adaptations in H5N1 viruses found in foxes and mink. Studies in wild birds and poultry have also identified mutations that may make the virus better at binding to “human-like receptors,” according to the WHO’s recent statement.

“Studies are underway to identify any changes in the virus that may help the virus to spread more easily among mammals, including humans,” the WHO said.

What about dogs and cats?

Although it’s rare, both dogs and cats can catch bird flu. Earlier this summer, veterinary officials reported that at least 16 cats died in Poland and tested positive for H5N1 bird flu, according to the Netherlands-based news agency BNO News. It’s the first time avian influenza has been linked to a large die-off in cats.

Influenza in pet dogs and cats is, in most cases, linked to the consumption of sick birds or infected raw meat. Last spring, for example, a pet dog in Canada died from H5N1 after chewing on a dead goose. So a simple way to protect your animals is to keep them indoors or on a leash. You should also avoid feeding them raw meat from game birds or poultry, according to the government of Canada.

Keep an eye out for symptoms of bird flu, which include fever, exhaustion, pink eye, lack of appetite, difficulty breathing or other respiratory symptoms, and neurological signs (such as tremors or seizures), per the Canadian government.

Is there a human vaccine for bird flu?

Yes. The US government has a stockpile of vaccines, including those specifically for H5N1, according to the CDC. “Since flu viruses change constantly, CDC continues to make candidate vaccine viruses (CVVs) as needed,” the CDC states.

Meanwhile, companies that manufacture vaccines say they could quickly make millions of flu shots, Reuters reported. Last spring, three companies including GSK and Moderna told the news agency that they’re developing vaccines designed to strengthen the human immune system against the particular type of HPAI spreading today.

“It’s not Covid,” Puryear said, referencing how there were no coronavirus vaccines stockpiled when the pandemic hit. “In theory, we should be able to respond quickly if this becomes an issue.”

One big caveat: Should we need bird flu vaccines, a large portion of them will likely go to rich countries first, leaving more vulnerable nations at risk.

What about a vaccine for birds?

Yes, there are some effective bird vaccines for H5N1 on the global market, as my colleague Kenny Torrella has written; countries including Egypt, Mexico, and Indonesia are already using them.

In the past, the USDA has licensed a handful of bird flu vaccines, though none of them are approved for the current strain, according to the American Medical Veterinary Association. Still, the USDA hasn’t approved them for actual use in mass vaccination campaigns on farms, Torrella writes, due to something called the “DIVA problem”:

DIVA is short for “differentiating infected from vaccinated animals” — the challenge of identifying whether a bird is actually infected with avian influenza, or just has avian influenza antibodies after vaccination. Countries fear that importing eggs or slaughtered meat from vaccinated birds in countries where the virus is circulating could inadvertently spread it within their own borders by introducing the virus to wild or domesticated animals through discarded raw meat.

That means that big poultry exporters like the US — which sends 18 percent of its poultry abroad — don’t vaccinate, for fear they’ll miss out on a huge part of their revenue: international trade.

That said, the USDA began testing several potential bird flu vaccines in April to see how well they might respond to the current strain of influenza virus. Vaccines could be commercially available within 18 to 24 months, the agency said in April. Several other countries are also testing influenza vaccines for birds.

Separately, USDA has approved the emergency use of a bird flu vaccine for the critically endangered California condors “in an attempt to prevent additional deaths of these birds.” Veterinary officials will start vaccine trials for condors later this month, according to Joanna Gilkeson, a spokesperson for the US Fish and Wildlife Service.

How can I help?

If you have pets, prevent them from interacting with infected birds. That means keeping your cat indoors and your dog on a leash.

You can also help monitor the outbreak, especially if you already spend a lot of time outdoors looking for wildlife. Birders are “the eyes and ears of this panzootic,” Nichola Hill, an infectious disease ecologist at the University of Massachusetts Boston, told Vox earlier this year. “I don’t think they’re being harnessed enough.”

While people are used to photographing beautiful wildlife on their phones and uploading it to platforms like iNaturalist, right now it’s more important that they document dead birds, she said. (iNaturalist actually has a webpage exactly for this purpose.) If you find a dead wild bird and are not sure what to do, check out this one-pager from the USDA.

Tips, comments, or questions? Reach me at benji.jones@vox.com.

Keren Landman and Kenny Torrella contributed reporting.