Since the virus was identified in humans three years ago in China, 80 of the 150 people infected have died from the disease. Fears of its spread has led to the slaughter of 50 million birds. WHO officials warn that H5N1 could trigger a global pandemic.
It is rare for avian influenza to move from birds to humans. World Health Organization spokesman Richard Thompson says infection comes directly from contact with sick poultry or from feces from diseased birds mixed in the soil.
He says those infected so far had been exposed to poultry they didn't know was infected. Farmers or their families come along, and to prepare the animals for consumption they de-feather the birds. He says, "In the act of pulling the feathers off these dead birds, dust flies up, and we believe that inhaling this dust has led to most of the infections in humans of the virus."
Worldwide outbreaks of a single disease are rare but reoccurring events, having been recorded historically every 10 to 50 years. WHO's Richard Thompson says vigilance - not panic - is in order. "There were three pandemics in the last century. Most people alive today lived through two of them without really noticing them. This wasn't a major trauma to societies or economies or even to health systems. So pandemics can occur which have relatively minor effects."
Others, like the 1918 Spanish Flu pandemic, killed 40 million people. No one can predict whether H5N1 will trigger such a pandemic, and - if it does - how long it would last or how deadly it would be.
For that to happen, experts say the virus must first mutate, or change so that it could pass easily among humans. Thompson says one way that might happen is if a person with the common, seasonal influenza virus were also infected with the H5N1 strain. "It is possible that the two viruses could mingle in one cell and from that single cell out would emerge a hybrid virus, a virus that had the characteristic of human influenza moving easily from person to person as well as the avian influenza characteristic being unrecognizable to the human immune system."
Thompson says H5N1 - like other strains of influenza virus - is also highly unstable, and natural changes could give it the ability to move efficiently from one person to another. Thompson says a pandemic outbreak is identified by how the virus attacks people. "If, for example," he says "we started to see clusters of human cases - not family members who may have had common environmental exposures, but rather casual contacts - [like] an ambulance driver who took a patient to the hospital or a nurse who had been tending to a patient. That would be a signal to us that this is the kind of clustering of cases that might signal the beginning of a pandemic."
For now, though, the H5N1 virus has apparently not yet passed from person to person, so only those who come into contact with infected birds are at risk. Thompson says the first line of defense is close observation by those raising or living around poultry. "This is the kind of surveillance system that really allows us to first identify how far the disease is spreading and then to rapidly move in and cull birds that might have been exposed and to clean areas to reduce the possibility that humans can become infected even from the droppings of these birds," he says.
WHO is also calling on governments to increase surveillance for early detection and to develop emergency plans to help contain the spread of a highly pathogenic virus.