The world is in the middle of a bird flu epidemic.
Hundreds of thousands of wild birds and millions of domestic birds have died from the H5N1 strain of the disease.
The disease of bird flu has been around for a century. It flares up in the fall.
"It came from ducks in Europe and Asia and spread to other birds," says Paul Digard, a professor at the Edinburgh University.
The H5N1 virus was first reported in China in 1996 and has since broken out occasionally.
The virus has persisted for a long time.
The H5N1 strain of bird flu can be spread through droppings, saliva, or contaminated feed and water within a matter of days.
The current wave of bird flu is the worst in history.
"A hundred and sixty million domestic birds worldwide have been killed by this virus, or have had to be culled by farmers to contain it," says Professor Iqbal.
There are 100 million domestic birds in the US.
Eggs are in short supply in the shops in western European countries, and there are fears of a turkey shortage at Christmas.
Half of the 1.3 million turkeys produced for Christmas in the UK have died or been culled due to bird flu.
Sea birds have been hardest hit by bird flu this year.
Professor Iqbal said the current virus has affected 80 bird species. In Scotland, it has killed 40% of the skua population.
Dr Louise Moncla is a veterinary expert at the University of Pennsylvania.
She says that the outbreak started in Europe and spread to North America.
We are in the middle of an "unprecedented wildlife disease outbreak, the breadth and scope of which is staggering".
Scientists don't know why this outbreak is so bad. It's possible that the virus has changed to allow it to spread more easily from bird to bird.
Dr Nancy Beerens is an expert on bird flu at Wageningen Bioveterinary Research in the Netherlands.
She says that it's unlikely that the virus will disappear from the bird population again.
China has been vaccinating its birds.
It is hard to judge which birds have been made immune by the vaccine and which have not, so meat and eggs from vaccine flocks cannot be sold abroad.
According to Dr Maurice Pitesky of the University of California, Davis, there are strict export controls when a country decides to vaccine.
The governments of EU countries and North America have told their farmers to kill all the poultry in any flock that has been affected by bird flu.
Farmers in the UK and France have been told to keep their free range poultry flocks indoors to prevent them from getting sick from wild birds.
France and the Netherlands have begun trials of vaccines to try and stop the bird flu epidemic.
Humans have caught bird flu when coming into close contact with birds.
Prof Digard says that the current strain of H5N1 appears to be low risk.
He says that we need to keep an eye on how far it is spreading and get reports from vets.
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