In a few cases, exposure of children to chicken faeces when they have been playing in an area with free-ranging poultry is thought to have been the source of infection.
Swimming in water where the bodies of dead infected birds have been discarded or which may have been contaminated by faeces from infected ducks or other birds might be another source of exposure. Other possible sources could be contact with semi-domestic birds, such as pigeons, or the use of untreated bird faeces as fertiliser.
The route of human infection with the H5N1 bird flu virus is by close contact with dead or sick birds. This means that activities such as slaughtering, defeathering, butchering and the preparation of infected birds for eating are very dangerous.
Having said all this, at present, the H5N1 bird flu virus largely remains a disease of birds, as the virus does not easily cross from birds to infect humans.
Despite the infection of tens of millions of poultry over huge areas since mid-2003, fewer than 200 human cases have been confirmed in the laboratory.
For unknown reasons, most cases have occurred in rural households where small flocks of poultry are kept.
Again, for unknown reasons, very few cases have been detected in the groups thought to be at high-risk, such as commercial poultry workers, workers at live poultry markets, vets and health staff caring for patients without adequate protective equipment.
Many scientist are now doing research into the factors that might increase the likelihood of infection into humans.