Highly pathogenic avian influenza, commonly called bird flu, has increasingly spread to mammals and infected hundreds of people, raising concerns that it may lead to human-to-human transmission and turn into a new pandemic.
Cases of the disease in mammals have mostly been detected in the Americas and Europe.
Sheep were added to the list on Monday with the UK government announcing a first case had been detected at a farm in northern England.
Some of the mammals such as dairy cows and sheep are farmed and so interact closely with humans, increasing the threat of transmission, while others have much less contact with people.
Pigs represent a particular concern for the spread of bird flu because they can become co-infected with bird and human viruses, which could swap genes to form a new, more dangerous virus that can more easily infect humans.
