A virulent strain of bird flu has been discovered in three mute swans found dead at a bird sanctuary in Dorset.
The virus can cause illness and death in humans, but the chances of being infected are extremely low.
Most of the people who have developed symptoms are poultry workers, mainly in south-east Asia, who have come into intimate contact with infected birds.
What is bird flu?
Like humans and other species, birds are susceptible to flu.
There are many types of bird, or avian, flu.
The most contagious strains, which are usually fatal in birds, are H5 and H7.
There are nine different types of H5. The nine all take different forms - some are highly pathogenic, while some are pretty harmless.
The type currently causing concern is the "highly pathogenic" Asian strain of the H5N1 virus.
Scientists have discovered four different subtypes of H5N1, and there could well be more. However, all are deadly to birds, and can cause disease - and death - in humans.
However, it is important to stress that H5N1 is overwhelmingly a disease that affects birds - and not humans.
It is true that humans have been infected, but almost all have been poultry workers who have come into intimate contact with birds. H5N1 cannot pass easily from human to human.
Migratory wildfowl, notably wild ducks, are natural carriers of the viruses, but are unlikely to actually develop an infection.
The risk is that they pass it on to domestic birds, who are much more susceptible to the virus.
How do humans catch bird flu?
Bird flu was thought only to infect birds until the first human cases were seen in Hong Kong in 1997.
Humans catch the disease through close contact with live infected birds.
Birds excrete the virus in their faeces, which dry and become pulverised, and are then inhaled.
Symptoms are similar to other types of flu - fever, malaise, sore throats and coughs. People can also develop conjunctivitis.
Researchers are now concerned because scientists studying a case in Vietnam found the virus can affect all parts of the body, not just the lungs.
This could mean that many illnesses, and even deaths, thought to have been caused by something else, may have been due to the bird flu virus.
Is it possible to stop bird flu coming into a country?
There is no failsafe way of preventing its spread.
Experts say proper poultry controls - such as preventing wild birds getting in to poultry houses - are vital.
In addition, they say monitoring of the migratory patterns of wild birds should provide early alerts of the arrival of infected flocks - meaning they could be targeted on arrival.