Avian influenza is a disease caused by influenza A viruses that spread widely among wild birds, particularly aquatic birds, birds of prey and waterfowl, but also domestic birds such as poultry. In the United States, highly pathogenic avian influenza has been detected in about 6,000 wild birds and affected more than 58 million commercial poultry and backyard flocks since the start of 2022, according to data from the Centers for Disease Control and Prevention.

“It’s been the most serious outbreak for birds that we’ve had in the U.S. of these highly pathogenic avian influenza viruses in recent memory,” said Nicole Nemeth, an associate professor of pathology and head of the wildlife research and diagnostic service at the Southeastern Cooperative Wildlife Disease Study at the University of Georgia.

According to the World Organization for Animal Health, avian influenza viruses can survive for long periods in cold temperatures on surfaces such as farm equipment, which allows them to spread from farm to farm.

It is rare, but humans can become infected with bird flu if they come into close contact with infected birds — whether the birds are dead or alive — or with surfaces that may have been contaminated by an infected bird’s saliva or feces.

Although the virus has been detected in wild mammals such as red foxes, raccoons, opossums and skunks — probably from eating infected wild birds — experts said the virus poses a low risk to humans. In fact, since the start of the current outbreak, only one human case has been reported in the United States.

People who work closely with infected poultry are at an increased risk of becoming infected.

Infections could range from mild cases such as conjunctivitis — an eye infection that could occur after handling contaminated material and then touching the eyes — to more serious but rare respiratory infections, experts said.

The virus typically doesn’t infect the human respiratory tract, because humans don’t have the receptors in their throats, noses or upper respiratory tracts that are susceptible to the current bird flu strain.

A person would need to breathe in a large amount of the virus — by sweeping up and inhaling infected fecal matter deep into the lungs, for example — to develop a respiratory infection from the virus, said William Schaffner, a professor of infectious diseases and preventive medicine at Vanderbilt University.

“In those circumstances, the virus can initiate an infection in an occasional human and quickly develop into influenza pneumonia,” he said, and then “the fatality rate is very high.”

Human-to-human transmission of the virus is very rare, Schaffner said.

“If that happens, that human being is usually a very close caregiver of the sick person,” he said.

Some people who are infected may not experience symptoms, according to the CDC. Others may have mild symptoms such as conjunctivitis or flu-like symptoms — fever, cough, sore throat, body aches, headaches, fatigue and, in more serious cases involving pneumonia, trouble breathing.

The symptoms of bird flu depend on the strain of the virus with which each person becomes infected. The strains that have caused most of the human infections in the last 25 years are H5N1 and H7N9, the CDC said.

Bird flu infection is diagnosed in a lab, usually by swabbing someone’s nose and throat.

People who contract bird flu are typically treated with supportive care and, in serious cases, with ventilators to help them breathe. There are also antiviral medications that are effective at treating the current strains, Schaffner said.

Yes. Federal authorities have vaccines against A(H5N1) and A(H7N9) bird flu viruses that could be used if these viruses were to start spreading among the human population, the CDC said.

Yes, it can be fatal, mainly when the virus gets into the lungs and causes influenza pneumonia, but that is rare.

According to the World Health Organization, between January 2003 and 2023, there were 868 laboratory-confirmed cases of human infection with H5N1 globally. Of those who were infected, 53 percent — or 457 — died. Between 2021 and 2022, six confirmed cases of human bird flu infections were reported to the WHO, and two of those infected died.

While these numbers might seem scary, experts caution that the risk to the general public is low and that there is no evidence of sustained human-to-human transmission.

Each time there is highly pathogenic avian influenza outbreak, it triggers concerns that the virus could mutate to infect humans more readily and start spreading from person to person.

That happened with swine flu in 2009, when pigs became simultaneously infected with avian influenza and human influenza. The two viruses exchanged their genetic material inside the pigs, allowing the bird flu to use the genetic blueprint from the human flu to spread among people — “a statistical phenomenon,” Schaffner said.

Such a pandemic cannot be predicted because this exchanged of genetic material is a random event.

“If anything, the odds are against it,” Schaffner said, noting that bird flu strains are circulating all the time and do not pose a risk to humans. Although this strain has infected some mammals — including mink, causing an outbreak at a Spanish farm — “that doesn’t necessarily mean it’s going to pick up the capacity to spread to humans,” he said.

Sign up for the Well+Being newsletter, your source of expert advice and simple tips to help you live well every day

Dogs have contracted avian influenza strains in the past, said Carol Cardona, chair of avian health at the University of Minnesota. But because the family dog or cat is not typically in contact with infected birds like wild animals are, their risk is low, she said.

Still, experts warned against letting dogs or cats eat dead birds for a variety of health reasons.

“If you are keeping your cat indoors and you’re keeping your dog on a leash, I don’t see any reason that you would be expecting to see an infection,” Cardona said.

No. Humans cannot become infected by eating eggs or fully cooked poultry. However, according to the CDC, “uncooked poultry and poultry products (like blood) could have been the source of a small number of bird flu virus infections in people in Southeast Asia.”

The CDC advises that “it is safe to eat properly handled and cooked poultry in the United States.”

Still, on commercial farms, Cardona said infected poultry — and other chickens or turkeys that are exposed to those infected poultry — are typically killed to protect the U.S. food supply. “That’s why there’s so much death associated, in many ways,” to the current outbreak, she said. “It’s good to know that they don’t make it into any food products. They don’t even make it into your pet food.”

People who own their chickens should be vigilant, experts said.

Backyard chickens could come into contact with wild birds either directly or through their bodily secretions, putting them at risk for potential infection. Experts recommended keeping backyard chickens inside an enclosure to keep them away from any migratory birds that may be carrying the disease.

Chickens that become infected may become unusually quiet, stop producing eggs or produce fewer of them and develop respiratory symptoms and diarrhea before they start dying.

People who have backyard chickens should still take precautions and avoid touching infected chickens or their eggs, or contaminated surfaces, experts said.

The outbreak drove up the price of Thanksgiving turkeys in November, but its impact continues to affect egg prices, which is dealing with a number of supply chain issues as well. According to the Agriculture Department, the bird flu has wiped out more than 44 million egg-laying hens, or roughly 4 to 5 percent of production.

Some locations have been more affected than others, but there is no place that is entirely risk free, said Munir Iqbal, head of the avian influenza virus group at the Pirbright Institute in England. “The virus is very much endemic now in wild birds and the wild birds are going all around the world, so therefore the threat to the poultry has become constant and unpredictable,” Iqbal said.

Highly pathogenic avian influenza was first identified in geese in Guangdong, China, in 1996. It spread widely in Asia between 2003 and 2005, mutating into different strains that have since caused periodic outbreaks around the world.

China, Egypt and some other nations that experienced prior deadly outbreaks of bird flu have also focused on vaccinating flocks of birds to decrease bird mortality. But vaccination for bird flu is also costly, and there is no universal agreement on whether it is an effective strategy to manage outbreaks.

The United Kingdom, where systematic vaccination of domesticated birds is banned in most cases, states in its official guidelines that vaccines are too difficult to administer properly over time, have not been proven to work in all affected bird species and are complicated to develop and adapt because the virus mutates so quickly.

Amid the current H5N1 outbreak, some European countries, including France and the Netherlands, are testing the effectiveness of bird vaccination.

In the United States and Europe, authorities monitor bird populations and, when they detect infections, cull flocks, including healthy birds who may have been exposed. This is costly because many governments compensate farmers when their healthy birds are killed.