Scientists often debate where the next pandemic will come from and what virus will cause it, but they tend to agree on one thing: At some point, the world will see another flu pandemic. “If you go back in history, there are a lot of flu pandemics,” said Jennifer Nuzzo, professor of epidemiology and director of the Pandemic Center at Brown University School of Public Health. “Pandemics happen somewhat regularly.” 

Of particular concern are the strains of flu that jump from animals to people and cause severe illness and death. That’s why H5N1, a strain of avian influenza or “bird flu,” is raising alarms in the United States. It’s rare for people to get H5N1, with 954 deaths documented over the past two decades, but among those with known infections the global mortality rate is 49 percent. Even if that rate were halved, Nuzzo said, it would be a major emergency: “25 percent mortality is like nothing we've seen. COVID is like 1 percent.”

H5N1 was first detected in 1959 on a Scottish chicken farm. But it only gained notoriety decades later, in 1996, when it began spreading from a goose farm to people in Guangdong, China. That outbreak proved deadly, claiming the lives of half of the people who tested positive. And it vaulted H5N1 into the international spotlight.

Realizing how deadly H5N1 was, and how easily it spread among birds, is what made US officials start preparing for a flu pandemic in the early 2000s. Since then, we’ve had sporadic outbreaks of H5N1 along with other avian influenzas. But what’s happening now is a different story.

Where do we stand on the current outbreak of avian flu?

The strain of H5N1 currently making headlines first appeared in North America in late 2021, and it has been jumping species in novel, worrying ways. Birds migrating to this continent from Europe brought over an avian flu virus able to infect mammals, such as seals. Last year, the virus then appeared in dairy cows, and since then in at least 70 humans—most of whom had contact with cows or poultry—showing that the virus is getting better at spilling over into new species. The US is now the global hot spot of known human infections.

The threat level has gone up in recent weeks. In January, a Louisiana man was the first person in the US to die after being infected with H5N1, and in February, two more people were hospitalized with bird flu. “We've doubled the number of people hospitalized for this virus in the last few weeks,” Nuzzo said. “Granted, it's from two to four. That's a small numbers jump, but it's still a doubling of the hospitalizations.” It’s still not clear why hospitalizations are going up, she said.

Also in February, there were two new detections of spillovers into dairy cows in Nevada and Arizona, with more mutations discovered and concerns mounting that the virus is now endemic, or circulating regularly, in cows.

Why are experts so worried about flu?

Flu is a particularly concerning virus because it is very good at mutating. Different flu variants can also combine with each other in a process called reassortment to make new, potentially more dangerous, strains—this ability goes up exponentially if the flu becomes endemic. 

“The virus has tricks up its sleeve to evolve and adapt,” said Christopher Heaney, an associate professor focused on environmental health, epidemiology, and international health at Johns Hopkins University. It is “demonstrating changes, adaptations, that are of significant concern for us to be aware of and to be cautious about.”

Avian influenza, or bird flu, is a disease caused by avian influenza A viruses. H5N1 is a strain within that family. Avian flu A viruses easily infect birds but tend to be less effective at infecting humans and other species, although it's not impossible. Bird flus are usually sorted into two categories—low and high pathogenicity—referring to how sick a virus makes poultry. H5N1 is a highly pathogenic bird flu.

On the rare occasions when highly pathogenic avian influenza does infect people, “it can have a high consequence—severe implications for people,” Heaney said.

Can I get bird flu from eating eggs?

Bird flu spreads through contact with animals and raw animal products. Most of the known cases in the US have been in dairy and poultry workers, although in two cases, it’s not clear how the patients were infected. There have also been cases of people sickened after contact with backyard flocks, including the Louisiana patient who died. To protect yourself from bird flu, avoid contact with wild animals, especially animals that appear sick or to exhibit odd behaviors, and wash your hands after touching raw food products. Bird flu seems to spread even with fleeting contact, such as a splash of raw milk on a farmworker’s face, experts say. Other precautions, such as wearing a mask, can also protect you from acquiring influenza viruses, especially if you are cleaning out your chicken coop. 

Unpasteurized milk can have very high concentrations of H5N1, and a study from the US Department of Agriculture also found traces of bird flu in meat that was cooked rare; the virus was inactivated in fully cooked meat. The processes of pasteurization and cooking subject raw ingredients to high enough temperatures to kill many pathogens, including this strain of bird flu, and in general it's wise to only consume pasteurized milk and fully cooked eggs and meat. 

Although physical contact with food is a worry, there haven’t been any known cases of bird flu among people from ingesting tainted food, but officials have warned that it is possible. Meanwhile, mammals such as cats are getting very sick from H5N1 after drinking raw milk and eating raw meat.

Can I get tested for or vaccinated against bird flu?

If you believe that you may have bird flu, you can ask your doctor to write a referral to a commercial lab that can test for it, such as LabCorp or Quest Diagnostics. You should mention to your doctor why you think you have bird flu, especially if you have been in contact with animals or if you consume raw food products. It’s important to note that the people most at risk are frequently not being tested. Farmworkers worry about losing their jobs or taking time off from work if they test positive.

The US has licensed bird flu vaccines in its national stockpile, but they are not yet being recommended for use in people, nor are they commercially available.

Getting a seasonal flu vaccine likely won’t help much with an H5N1 infection, but it can reduce the risk of flu reassortment and takes some pressure off of health systems. By keeping preventable hospital admissions at bay, institutions are better poised to absorb emergency situations, such as a human infected with H5N1. It also makes it easier for scientists to find rare variants like H5N1 circulating among people when there are fewer positive flu samples to test.

“There are a lot of reasons to get a seasonal flu shot, particularly this year, one of the worst flu seasons that we've seen in a decade,” Nuzzo said. A person who is co-infected with H5N1 and seasonal flu “is particularly worrisome because that's probably the most likely scenario to create a pandemic flu virus,” Nuzzo said. “Worrying about H5N1 20 years ago is why we had a pandemic preparedness plan, why we have things like H5N1 stockpiled vaccines, why we have things like tests and Tamiflu.”

What roles do factory farms play in the bird flu outbreak?

Large industrial farms known as concentrated animal feeding operations (CAFOs) keep thousands of animals in close quarters, making them a significant breeding ground for viruses and mutations of those viruses.  

“Think about the scale, the numbers of animals within which there are millions to billions of viral copies, which are all reassorting, every second, every minute of every day,” said Heaney. “The challenge with the production of so many animal species in confinement conditions is that that represents such a high-magnitude reservoir for a virus like this.”

That’s why it’s especially important for officials and farm owners to prioritize and protect agricultural workers who frequently come into contact with animals and who may be at risk of getting very sick and passing the virus on to others.

“Workers live lives. They go to work; they go home; they have families; they live in communities,” Heaney said. And if we start seeing spread in communities, he asked, “would that be too late at that point” to stop a pandemic?

Does climate change play a role in flu outbreaks?

Our changing climate, and the ways we reshape land with agriculture and deforestation, make new pandemics a greater risk. As humans come into closer contact with more animals, it increases the likelihood of spillover, which happens when a virus moves into a new host, which is what happened when H5N1 moved from birds to cows to people.

“The conditions that give rise to new viruses that can go on to cause pandemics—those conditions are increasing,” Nuzzo said. ”So the cadence that we saw in previous centuries could very well increase due to changing environmental conditions that make it easier for new viruses to spill over.”