After bird flu wreaked havoc in Europe and started spreading, health experts had been bracing for an outbreak in the US. The outbreak is now in full swing, and it shows no signs of going away. The US is currently going through the longest and deadliest avian influenza outbreak on record, with over 57 million birds killed by the virus or culled since a year ago, according to official data, with many fearing a potential spread to humans.

It’s the worst impact the poultry industry has seen since 2014-2015, when over 50 million birds died. Back then, the outbreak also started in the winter and it was over by the following June. Now, the current outbreak has lasted through the summer and has surged again. Birds are being culled all over to try to stop the virus from spreading, but the disease shows no signs of stopping.

Unlike the previous outbreak, this one is driven by wild birds and not by farm-to-farm transmission — yet. Early infections in commercial flocks started in the intersection of the Central and the Mississippi flyways of migratory birds. As birds traveled, the virus quickly expanded. There are now 50 states with bird flu in wild birds, the CDC said.

The H5N1 strain of avian flu is responsible for the deaths of millions of birds in the US. The strain was first identified in domestic waterfowl in China in 1996. Since then, it has re-emerged periodically before naturally petering out. The rate of reproduction in birds is high. One can infect up to 100 others through saliva and other bodily fluids.

The flu outbreak in the US has sent egg prices through the roof. Since it was first detected in January 2022, the price of a carton of a dozen eggs rose from about $1.79 to 4.25 – a 137% increase, according to the Bureau of Labor Statistics. There have also been cases of people trying to smuggle in egg cartons, as reported by local media.

This season’s outbreak is also the worst in Europe’s history. There have been 2,500 outbreaks on farms across 37 countries from October 2021 to September 2022, with about 50 million birds culled across the continent – especially in France. Japan also said last week it plans to cull over 10 million chickens at risk of exposure to the virus.

The health risks

Health experts are closely tracking avian flu transmission around the world. US government officials and the World Health Organization have said the risk for human infection is low, asking people to avoid unprotected contact with wild or domestic birds that look sick. If the contact can’t be avoided, they should take more precautions.

The WHO reported last week that a healthy Ecuadorian girl had been hospitalized with the virus. It’s not clear yet how she got the virus, but a poultry her family had recently bought died without evident cause. If the bird flu makes a sustained jump to humans, it could start an influenza pandemic, epidemiologist Rajiv Chowdhury told Fortune.

Between 2003 and 2022, the H5N1 strain has turned up in 865 humans in 21 countries, causing 456 deaths, according to WHO data. While most of the cases were linked to people being in touch with infected poultry rather than human-to-human transmission, health experts have told The Guardian the need to remain vigilant.