Over the last three years, a highly contagious, often deadly form of bird flu has taken a staggering toll on animals around the globe.

The virus, known as H5N1, has infected birds in more than 80 countries. It has infiltrated big commercial poultry farms and tiny backyard henhouses, affecting 72 million farmed birds in the United States alone, according to the Department of Agriculture. It has struck a wide range of wild bird species, killing gulls and terns by the thousand. And it has turned up repeatedly in mammals, including foxes, skunks, bears, cats, sea lions and dolphins. (It has also caused a small number of deaths in people, primarily in those who had close contact with birds. The risk to the general public remains low, experts say.)

The virus is not done yet. It is surging again in Europe and North America and causing mass animal mortality events in South America. It also appears to be spreading in the Antarctic region for the first time.

“It continues to be unprecedented,” said Thomas Peacock, a virologist at the Pirbright Institute in England. “By several measures, we’re at the worst it’s ever been, particularly in terms of geographical spread, how widespread it is in birds and how many mammals are getting infected.”

In Europe, however, where the virus has been circulating the longest, early signs suggest that this winter may not be as bad as the last few, Dr. Peacock said. And there is very preliminary evidence that some wild birds might be developing immunity to the virus.

The virus is expanding into new territory.

The current version of the virus has spread around the world with astonishing speed. After emerging in 2020, it quickly began causing outbreaks in Europe, Africa and Asia. In late 2021, it showed up in North America, storming through Canada and the United States. In the fall of 2022, the virus appeared in South America, spreading down to the tip of the continent in mere months.

This rapid southward spread prompted concern that the virus would soon reach Antarctica, which provides critical breeding habitat for more than 100 million birds. And in October 2023, the virus was found in the Antarctic region for the first time, detected in brown skuas on Bird Island, South Georgia. Since then, scientists have identified additional confirmed or suspected cases in gulls and petrels as well as in elephant seals and other animals in the region, according to the Antarctic Wildlife Health Network.

Although the virus has not yet been reported on the Antarctic mainland, scientists said they were expecting that news to come any day now. “It probably is already in Antarctica, but it hasn’t been picked up,” Dr. Peacock said.

Many of the birds and marine mammals in the region are already struggling to survive in the face of climate change and other threats. And because Antarctica has never been hit by a highly pathogenic bird flu virus before, its wild animals could be especially vulnerable to this one, scientists say.

Seasonal patterns may be emerging.

In the United States, summer provided a respite from what had already become the worst bird flu outbreak in the nation’s history. Between May and September, the nation logged just several small outbreaks in poultry, and cases in wild birds tapered off.

“We breathed a sigh of relief for a number of months when things really quieted down,” said Rebecca Poulson, an expert on avian influenza at the University of Georgia. “But it’s back. Or maybe it never left.”

Since the beginning of October, the virus has hit more than 1,000 poultry flocks in 47 states; 12 million farmed birds have been affected, according to the U.S.D.A.

Europe has documented a similar pattern, with virus detections increasing sharply in late October, according to a recent surveillance report from the European Center for Disease Prevention and Control.

Although the virus is still relatively new, these seasonal cycles might be here to stay. “My gut would say it might be part of the new normal,” Dr. Poulson said.

Hot, humid weather is not traditionally conducive to the spread of flu viruses, and many birds are stationary in the summer, spending those months on their breeding grounds. In the fall, many birds begin migrating and avian populations swell with young birds that have little exposure to the flu. All of these factors can fuel autumn surges. (The virus can also flare up in the spring, when birds migrating in the other direction congregate at high densities.)

Immunity remains a wild card.

Now that the virus has been circulating for several years, critical questions have arisen regarding immunity: Do birds that survive a brush with the virus gain some immunity against it — and could that dampen the ferocity of these outbreaks?

There is little data so far, but in one recent study, scientists found potential signs of immunity in northern gannets, a seabird species that suffered heavy losses in H5N1 outbreaks in 2022. “This is encouraging, particularly for species with threatened populations,” said Diann Prosser, a research wildlife ecologist at the U.S. Geological Survey’s Eastern Ecological Science Center.

More anecdotally, in Europe, some of the bird species that were hit hard in previous years do not seem to be dying off at the same rate, Dr. Peacock said.

Scientists said they expected that birds that survived infection would develop some degree of immunity to the virus. But what that means for the future of the panzootic — the animal version of a pandemic — will depend on a variety of factors that are harder to pin down, such as how robust that immune protection is, how long it lasts and how well it holds up against a virus that has been evolving rapidly.

“I would expect that development of immunity within the wild bird populations would affect the trajectory of the panzootic, while the specific path is hard to predict,” Dr. Prosser said.

Outbreaks in mammals are causing concern.

Although the virus is a threat primarily to birds, it has been showing up with unusual frequency in mammals, especially in wild scavengers like foxes. Many of these cases have probably been dead-end infections, in which mammals contracted the virus after eating infected birds and then died without passing the virus on.

But some larger outbreaks have caused concern. In the fall of 2022, the virus hit a mink farm in Spain, and over the last several months it has been detected in numerous fur farms in Finland, which house mink, foxes and raccoon dogs. In Peru, H5N1 has been linked to mass die-offs of South American sea lions.

Viral samples taken from some of these animals have contained mutations that are known to make the virus better adapted to mammals. Although it is not unusual to see those mutations pop up when mammals are infected, these findings, combined with the size and speed of the outbreaks, have been worrisome. “It looks like there was probably mammal-to-mammal transmission in at least a couple of cases,” Dr. Peacock said.

Although human infections remain rare, a version of H5N1 that spreads more easily among mink or sea lions could also spread more easily among humans, potentially setting off another pandemic, scientists worry.

Several curious outbreaks in cats have also been reported this year. One, at a cat shelter in South Korea, was linked to contaminated food, which has also been suggested as a potential cause of cat infections in Poland. Although it is not clear whether the virus spread from cat to cat, viral samples did show signs of mammalian adaptation. And every infection of a mammal provides more opportunities for the virus to mutate and evolve, posing risks not only to humans but also to other wild creatures.

“We’re worried about these viruses jumping into mammals and then maybe more specifically into humans,” Dr. Poulson said. “I just always like to point out that wildlife is important for its own sake. And this has proved to be a really devastating virus to mammalian and avian species.”