Anyone who did any baking this holiday weekend might have noticed that eggs have become more expensive. The price of a dozen grade A eggs jumped from $1.82 at this time last year to almost $3.60 now, according to the Bureau of Labor Statistics.

That’s partly the effect of rising chicken feed prices. But the real culprit is bird flu, which has infected over 57 million hens in the U.S. this year. The disease has been flaring up throughout the year, starting around springtime.

“That was the first time we had egg prices spike. And then we had recurring outbreaks of it,” said Daniel Sumner, a professor of agricultural economics at the University of California, Davis.

He added that eggs aren’t like steak or fancy wine, which people might cut back on if prices rise. For a lot of people, eggs are a staple. So when prices go up, people tend to buy them anyway.

“That drives the price higher and higher. And that has to happen because the supplies are more limited,” Sumner said.

The Agriculture Department said grocery stores have been cutting back on promotions. And those being done are limited to organic and cage-free eggs.

“So if you look at free-range and cage-free … some of those producers are much smaller farms, so they’ve not been affected as much by bird flu,” said Phil Lempert, a food industry analyst and editor of the Supermarket Guru website.

But Lempert added that those make up a tiny share of overall egg production — and tend to be even more expensive.