Here are some key facts about avian influenza, after cases of H5N8 bird flu were detected on farms in Germany and the Netherlands:
Infectious viral disease of birds
* Bird flu, known as avian influenza, is an infectious viral disease of birds that infects wild water fowl such as ducks and geese and can spread to domestic poultry.
Read: Current bird flu viruses could cause pandemic
* Bird flu viruses are divided into two groups based on their ability to cause disease, or "pathogenicity". Highly pathogenic bird flu spreads rapidly, may cause serious disease and has high death rates in birds. Low pathogenic bird flu can cause mild disease that may be undetected, or cause no symptoms at all in some species of birds.
Often spread through the water route
* Bird flu viruses are often spread through the water route – wild birds shed viruses in their faeces into lakes shared with other birds. The World Organisation for Animal Health (OIE) says animal feed can be another factor in transmission of the disease – if wild birds and others have access to the same feed – as well as equipment and clothing.
* Flu viruses have a relatively high mutation rate and their genome structure allows them to interchange genetic material fairly easily, meaning two or more strains could "mix" to create a new threat.
* Most bird flu viruses don't infect people, but some, such as H7N9 and the highly pathogenic H5N1, have caused serious human disease and deaths.
Read: 'Safe' bird flu strain infects human
* The H5N8 strain found in poultry in Germany and the Netherlands this month has never been detected in humans, but it led to the destruction of millions of farm birds in Asia, mainly South Korea, after an outbreak earlier this year.
First infected humans in 1997
* High pathogenic H5N1 bird flu first infected humans in 1997 during a poultry outbreak in Hong Kong. Since its re-emergence in 2003 and 2004, H5N1 has spread from Asia to Europe and Africa and has become entrenched in poultry in some countries, causing millions of poultry infections, several hundred human cases and many human deaths.
* H7N9 bird flu, a low pathogenic type, first infected three humans in China in March 2013. It has since infected more than 450 people and killed 175 of them, but no cases of H7N9 infection outside China have been reported to the World Health Organisation.
Read: Bird flu: how worried should we be?
* The majority of human cases of H5N1 and H7N9 have been associated with contact with infected live or dead poultry. There is no evidence the disease can be spread to people through properly cooked food.
Q&A of bird flu
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Image: Man in an overall plucking the feathers off a bird's corpse from Shutterstock.