Health experts meet on November 7 to draw up a global strategy to halt the spread of the deadly H5N1 bird flu and prevent it from developing into a human pandemic in which millions could die. Here are key facts about the transmission of the virus:

• H5 and H7 subtypes of the avian flu virus can be of either low or high pathogenicity. The discovery of an H5 type virus does not necessarily indicate the presence of H5N1.

• High pathogenic H5N1 is particularly deadly to poultry (it can kill an entire flock within hours) but less so for wild ducks and geese, which act as reservoirs for the virus, most of the time show no symptoms and can fly long distances with it.

• All birds are liable to infection from avian flu viruses as are some other animal species such as pigs although this is less common.

• Transmitted via nasal/oral secretions and faeces.

• Migrating wildfowl believed to be responsible for the spread of the virus from Asia and Siberia to Romania and Turkey. But trade in live poultry may have played a role in Asia.

• Relatively difficult to transmit from bird to human. Thousands of cases among poultry in Asia have resulted in at least 123 human cases and more than 60 deaths. It must also be noted this is a region where there is often close human contact with live poultry in backyard farms.

• Humans would have to be in prolonged close contact with an infected bird, in a confined space, as the virus can be carried in faecal dust or have direct contact with surfaces contaminated by infected droppings or secretions.

• Whilst the virus can exist in tissue, there is no evidence properly cooked poultry or eggs can be a source of infection. H5 and H7 highly pathogenic viruses are rendered inactive by heat (60 degrees Celsius/30 minutes) and by acid pH.

• In Asian cases, exposure to the virus is thought most likely dur-ing slaughter, defeatheri-ng, butchering, preparation of poultry for cooking.

• In 2003 a milder form of bird flu struck the Netherlands. Although it was a strain not normally dangerous to humans, some cases of conjunctivitus were noted and one veterinarian, who had prolonged close contact with infected birds, died.

• Of the few bird flu viruses that have crossed the species barrier to hum-ans, H5N1 has caused the largest number of severe diseases and deaths. It follows an unusually aggressive clinical course, with rapid deterioration and high fatality. Pneumonia and multi-organ failure are common.

(Sources: WHO), World Organisation for Animal Health (OIE))