viruses can mutate

Influenza viruses can mutate
Influenza viruses that infect animal species have the ability to mutate and also infect humans. The human immune system may have no defences against viruses that previously have only infected animals; therefore, infection with these viruses can result in more severe disease. It is also possible that the H5N1 virus may 'mix' with human influenza strains to form a new viral subtype not covered by the vaccine.

If the H5N1 virus were to mix with a human influenza virus, such a 'combined' virus could create a new human influenza virus with the capacity to spread rapidly. Health experts are concerned that, if unchecked, the current bird flu affecting South East Asia could become a worldwide pandemic.

In the past, influenza pandemics have caused global havoc. The worst influenza pandemic in modern history was the Spanish flu, which occurred in 1918–19 and killed up to 50 million people. Measures to contain the spread of current bird flu include identifying and culling affected poultry flocks, research into developing appropriate tests and vaccines, and rigorous quarantine practices.

How the virus is spread
It is thought that water birds such as wild ducks are the carriers of all avian influenza type A viruses. Indeed, influenza type A viruses of other animals, including humans, may have evolved from avian influenza strains. The viruses are carried inside the intestines and are distributed into the environment via faeces.

Migratory birds infected with the virus could potentially spread the bird flu to whichever countries they visit. Wild birds don't usually get sick, but transmit the virus to domesticated birds, such as chickens and turkeys, which can be killed by the infection.

Symptoms in birds differ according to the species, but can include diarrhoea, respiratory distress, swollen head and death. A sick bird sheds the viruses in its feathers, mucous, saliva and faeces.

It is thought that humans are at risk of infection if they are in close contact with sick birds. For example, a person may handle a sick bird, contaminate their hands with chicken faeces, and then forget to wash their hands before eating. They will then ingest the infected bird faeces. This is the most common way for a human to catch the bird flu.

The virus can also survive in raw poultry meat. There is no evidence that the current circulating strain of bird flu can be spread from human to human.

Incubation period
Although there have been too few human cases to determine the exact incubation period of bird flu, it would be expected to be from three to 10 days.