H5N1 is a specific type of influenza virus, predominantly harboured by birds, that was first detected on a goose farm in China in 1996.
The virus is highly pathogenic to birds, meaning that infections often cause extreme symptoms, including death.
There have been relatively few human infections detected globally over several decades but about half of those infected individuals have died.
The good news about H5N1 for humans is that it currently doesn't spread well between people.
The first reason that so much attention is being paid to bird flu right now is that currently H5N1 is causing the largest "bird pandemic" ever recorded.
A certain viral variant that arose in 2020, called H5N1 2.3.4.4b, is driving this outbreak.
In agricultural poultry flocks, if a few birds test positive for H5N1, the whole flock is killed regardless of symptoms.
The second reason for increased attention is that H5N1 is now infecting more bird and mammalian species than ever before.
The virus has also been detected in a broad array of wild birds and in diverse mammals.
As H5N1 infects more species, it also increases its geographical range and produces more viral variants that could have new biological properties.
Birds experience influenza as a gastrointestinal infection and spread flu predominantly through defecating in water.
By contrast, humans experience influenza as a respiratory infection and spread it by breathing and coughing.
Over the centuries, some of these avian influenza viruses have been passed from birds to humans and other mammalian species.
Avian flu viruses have evolved to infect cells of the intestine, while human flu viruses have evolved to infect cells of the respiratory tract.
Both human and bird influenza viruses use receptors called sialic acids that are common on the surfaces of cells.
Birds experience influenza as a gastrointestinal infection and spread flu predominantly through defecating in water.
This is because we don't know exactly what the genetics of the virus will be if it starts to spread well in humans.
Right now, the best way people can protect themselves from H5N1 is to avoid contact with infected birds.