The average price per dozen nationwide hit $4.15 in December. That’s not quite as high as the $4.82 record set two years ago, but the Agriculture Department predicts prices are going to soar another 20% this year.

Shoppers in some parts of the country are already paying more than double the average price, or worse, finding empty shelves in their local grocery stores. Organic and cage-free varieties are even more expensive.

Some grocery stores have even limited how many eggs shoppers can buy.

“It’s just robbery,” said Minneapolis resident Sage Mills, who bought eggs to bake a birthday cake last week. “Eggs used to be kind of a staple food for us, but now you know, you might as well just go out to eat.”

What is driving up prices?

The bird flu outbreak that started in 2022 is the main reason egg prices are up so much.

Anytime the virus is found on a poultry farm, the entire flock is slaughtered to help limit the virus’ spread. And with massive egg farms routinely housing more than 1 million chickens, just a few infections can cause a supply crunch.

The problem tends to linger because it takes months to dispose of all the carcasses, disinfect barns and bring in new birds.

More than 145 million chickens, turkeys and other birds have been slaughtered since the current outbreak began, with the vast majority of them being egg-laying chickens.

Cage-free egg laws in 10 states may also be responsible for some supply disruptions and price increases. The laws set minimum space for chickens or cage-free requirements for egg-laying hens. They’ve already gone into effect in California, Massachusetts, Nevada, Washington, Oregon, Colorado and Michigan. At a Target in Chicago on Monday, a dozen large conventional eggs cost $4.49 but a dozen large cage-free eggs were selling for $6.19.

Why is the virus so hard to control?

Bird flu is primarily spread by wild birds such as ducks and geese as they migrate. While it is fatal to a variety of animals, those species can generally carry it without getting sick, which offers the virus a chance to mutate and thrive.

The virus can be spread through droppings or any interaction between farm-raised poultry and wild birds. It’s also easily tracked into a farm on someone’s boots or by vehicle.

Unlike previous outbreaks, the one that began in 2022 didn’t die out in high summer temperatures.

The virus found another new host when dairy cattle started getting sick last March. That creates more opportunities for the virus to linger and spread and unlike poultry, cattle aren’t slaughtered when they get sick because they rarely die from bird flu.

More than five dozen people have also become ill with bird flu and one person died since last March. Nearly all of them worked around sick animals. Health officials haven’t yet found evidence of the disease spreading from person to person.

What is being done to stop the virus?

Farmers go to great lengths to protect their flocks.

Many poultry farms installed truck washes to disinfect vehicles entering their property and require workers to shower and change clothes before stepping inside a barn. They have also invested in duplicate sets of tools so nothing used in one barn is shared.

Some poultry farmers have even invested in lasers that shoot beams of green light in random patterns to discourage ducks and geese from landing.

Dairy farmers isolate any sick cattle and do additional testing before moving animals off the farm — especially if there has been a nearby outbreak or if the cows are being sent to a meat processing plant. The government is also testing milk.

Future vaccines might help, but it’s not practical to vaccinate millions of chickens through shots, and other countries might refuse to purchase meat from vaccinated birds.

Health officials emphasize that any sick birds or cattle are kept out of the food supply. Cooking meat to 165 degrees (74 Celsius) kills bird flu, E. coli, salmonella or anything else. Pasteurization also kills the virus in milk. Raw milk is the only food product linked to illnesses so far.

How much has the outbreak cost so far?

It is impossible to know how much farmers have spent to seal barns, build shower houses for workers or to adopt other biosecurity measures.

“Over the last five years, my small farm alone has spent hundreds of thousands of dollars on biosecurity,” said Minnesota turkey farmer Loren Brey. “But not only that, it’s the time daily that you’re attending to biosecurity.”

The U.S. Department of Agriculture has spent at least $1.14 billion compensating farmers for the birds they have had to kill. A similar number wasn’t immediately available for how much has been spent to aid dairies.

USDA spokeswoman Shilo Weir said the department also spent more than $576 million on its own response.

The prices of turkey, milk and chicken have also seen some pressure from bird flu.

Mike Vickers, a manager at Sentyrz Liquor & Supermarket in Minneapolis, said he can’t even stock any organic, cage-free or brown eggs right now and is limited instead to selling large or jumbo eggs. He understands the pain customers are feeling.

“It’s the first time in my life that I’ve ever had to be kind of embarrassed on what I’m selling eggs for,” he said. “And it’s not our fault. We’re paying today $7.45 for a dozen eggs. We’re selling for $7.59. We’re making $0.14. That’s doesn’t pay the bills.”