What to know about the deadliest bird flu outbreak in history

As of Monday, more than 52.4 million birds across the United States have died of avian influenza, according to the Centers for Disease Control and Prevention.

This makes the outbreak the deadliest in history, surpassing the 2015 outbreak that killed 50.5 million birds in 2015.

While the 2015 outbreak was mainly contained to poultry farms, the current outbreak has spread to nearly every state.

What is so-called bird flu and what does it mean for Americans?

What is avian flu?

Avian flu is a disease of birds, which occurs when they are infected with avian influenza Type A viruses.

Domestic poultry, including chickens and turkeys, can be infected and spread the disease as well as other bird and animal species, according to the CDC.

"There are multiple versions of influenza, some that infect humans, some that infect animals," said Dr. John Brownstein, an epidemiologist and chief innovation officer at Boston Children's Hospital and an ABC News contributor. "It's been something we've been monitoring for two decades now and it creates a real problem, not only for wildlife but it can have dramatic impacts on domestic stock."

He added, "Because this virus is so contagious, drastic measures have to be put into place to stop the spread and that ultimately results in the death of millions of birds."

Can people be infected with avian flu?

Avian flu viruses do not usually infect people but there have been rare cases.

Because the virus is shed through mucus, saliva and feces, most infections occur when a human comes into contact with these fluids or inhales them.

Symptoms can range anywhere from mild to severe and, sometimes, result in death, according to the CDC.

"We have to remind people the risk to humans is low, but at the same time, unprotected contact with birds that look sick can pose a risk," Brownstein said. "An additional layer is when you have this much virus spread, there's opportunities for mutation and this is where there's an opportunity for a version of this virus that could actually have deeper impact in the human population as well."

He said there is no evidence to suggest people can contract bird flu from eating poultry meat or poultry products, such as eggs.

Why is there an outbreak?

It's believed that the outbreak began from wild birds. In January, the United States Department of Agriculture announced a highly pathogenic strain of avian influenza was found in wild birds for the first time since 2016, mainly in North Carolina and South Carolina.

The birds then migrated, spreading the virus to farms, with the first outbreak confirmed Feb. 9 at a turkey farm in Dubois County, Indiana.

Since then, birds across 46 states have either died as a result of infection or been killed due to exposure of infected birds, according to the USDA.

By comparison, the 2015 outbreak only affected birds in 21 states.

What should people do?

The CDC recommends that people should avoid contact with wild birds whenever possible, especially because they can be infected with avian flu and not appear ill.

In addition, those who work directly with birds such as in zoos, in wildlife conservations centers at meatpacking plants or on farms -- or those who have backyard poultry -- should use protection.

"Right now you want to use protective equipment, like gloves and N-95 masks," Brownstein said. "Just like any virus, you want to avoid touching your mouth, nose, your eyes after contact with birds. It's important changing clothes if you've had any contact."

He also recommended that people receive their flu vaccine as soon as possible.

"It won't prevent infection from bird flu but can reduce the risk of getting sick," Brownstein said.