The world is going through its worst-ever outbreak of bird flu.
The highly infectious H5N1 strain is responsible for the deaths of hundreds of thousands of wild birds and millions of domestic ones. It is also being found in mammals.
What is bird flu and how deadly is it?
Bird flu is an infectious disease of poultry and wild birds that has been around for a century. It usually flares up in autumn before fading away in spring and summer.
"It originated amongst ducks in Europe and Asia, and spread to other birds," says Paul Digard, a professor of virology in the Roslin Institute at Edinburgh University.
The H5N1 virus, which is the most prevalent strain now, was first reported in China in 1996.
It can spread through entire flocks of domestic birds within a matter of days, through birds' droppings and saliva, or through contaminated feed and water.
The World Organisation for Animal Health (WOAH) has recorded almost 42 million individual cases in domestic and wild birds since the outbreak began in October 2021.
Almost 15 million domestic birds, including poultry, have died from the disease, and more than 193 million more have been culled.
In some European countries it has led to egg shortages in shops.
What's so unusual about this outbreak?
More wild birds than ever before have been killed by this outbreak - with sea birds being especially hard hit.
The current virus has affected 80 different bird species," says Professor Munir Iqbal, the head of the Avian Influenza Virus (AIV) group at the Pirbright Institute.
More than 40% of the skua population in Scotland, and thousands of Dalmatian pelicans in Greece have died.
Scientists are unsure why this outbreak is so much worse than others. It may be that the virus has mutated to enable it to spread more readily from bird to bird, or to hang around longer in the environment.
Dr Nancy Beerens, bird flu expert at Wageningen Bioveterinary Research in the Netherlands, which analyses suspected bird flu samples, says the virus may now be ubiquitous in wild birds.
"As the virus now has infected many wild bird species, it becomes unlikely that it will disappear again from the bird population," she says.
What's being done to tackle the outbreak?
China has been vaccinating its domestic poultry flocks.
However, other countries avoid this because it is hard to judge which birds have been made immune and which have not - and so the meat and eggs from vaccinated flocks cannot be sold abroad.
"There are strict export controls when a country decides to vaccinate," says Dr Maurice Pitesky of the School of Veterinary Medicine, University of California, Davis.
Governments in EU countries and North America have instead generally told their farmers to cull all the poultry in any flock in which bird flu has broken out.
Farmers in the UK and France have also been told to bring free range poultry flocks indoors, to stop them being infected by wild birds.
Despite the commercial drawbacks of vaccinating poultry, governments in France and the Netherlands have begun trials of vaccines to try and bring the bird flu epidemic under control.
How is bird flu passing to mammals?
WOAH has counted 119 outbreaks of the virus amongst mammals, although it says this is undoubtedly an underestimate.
Species affected include dolphins and seals, foxes and otters in the UK, grizzly bears in the US and mink in Spain.
It is believed the infected mammals had fed on dead or sick wild birds which had the virus.
There has been no evidence yet of the virus spreading between mammals.
The virus may also have mutated to infect the mammals more easily.
Bird flu 'spills over' to otters and foxes in UK
Is bird flu a risk to humans?
The World Health Organization (WHO) says 870 humans have been infected with avian flu over the past 20 years, and 457 have died.
These cases occurred when humans came into close contact with infected birds.
The World Health Organization says the further spread of the H5N1 virus will have to be monitored closely to see whether it is mutating into a form which can spread amongst humans.
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