What is often forgotten about bird flu is just how cruelly and violently it kills as many as half the people it infects - a fact moderated only by the speed with which they will die. Symptoms we normally associate with the run-of-the-mill flu - such as a sore throat or runny nose - will appear first, but that is where the similarities with the annual winter variety of influenza end. H5N1 is a deadly virus, spawned by bird incubators, and it has adapted to move from the upper respiratory system down into the lungs. From various human case studies completed so far, we know it causes rapidly progressing pneumonia, destroying the tissue and causing the air sacs in the lungs to fill with fluid. Victims are choked to death by the prevention of oxygen from entering the bloodstream. Death usually comes within a week. Millions could die and economies will be paralysed, but at least those who survive will be spared the psychological trauma the world has not seen since the Black Death, which wiped out one-third of the earth's population from 1347 to 1350. During that pandemic, blood and pus oozed from egg-sized swellings in victims' groins and armpits, with their sweat and blood, breath, urine and excrement smelling foul. The way bird flu kills chickens is equally merciless. Medical descriptions from farms from the United States to Thailand describe how it overpowers not only their lungs but all living tissue in their body. There is massive haemorrhaging from blood vessels, the eyes, the beak and anus of the bird. Less is known about the way it attacks humans. Despite the fact that more than 60 people have now died from H5N1 since the latest outbreak that began in 2003, there have not been enough thorough autopsies on victims for scientists to detail a specific pattern the disease takes when it is inside the human body. But an ominous precedent is in place, according to University of Hong Kong microbiology professor Malik Peiris, who says the virus has been found in the brain of one human victim from the world's first outbreak in 1997 in the city. That is when the virus first jumped the species barrier. 'But it is still unclear if this [effect in chickens] happens in humans and to what extent it happens because there is still not much known about the flu's progress,' he said. Scientists believe they have evidence that bird flu can already pass between humans in close contact with each other, but it has not been confirmed. How it finds an easier and swifter way to do this will largely depend on chance. The worst-case scenario is a mutated virus that will be at its most deadly if it manages to transfer exactly as it exists in birds to humans - as the virus did in 1918. But if it does mutate with a normal human flu, the virus will not be as virulent, says Professor Peiris. This is what happened with the flu in 1967-68 - the so-called Hong Kong flu - which killed 'only' 1 million people. The worst-case scenarios - those that governments across the globe are preparing for - estimate a body count of 150 million. The Spanish flu of 1918 to 1919 killed up to 100 million people, but came when the planet was dealing with the trauma and upheaval of the first world war and information about the flu was not so readily available. The world is smaller now and more densely populated. And although we have better diets, anti-viral drugs and global institutions such as the World Health Organisation mapping out action plans, we know little of what will happen if bird flu is not contained and becomes a pandemic. More frightening is that Spanish flu had a mortality rate of about 5 per cent. H5N1 has so far killed a staggering 55 per cent of those it has infected. Post-Sars Hong Kong is better prepared than anywhere, according to the WHO, but if the bird flu virus is not immediately spotted and contained, no one is safe. 'How long does a plane take to travel from China to New York,' as WHO spokesman Peter Cordingly chillingly put it. So what is the likely response from wealthy nations towards those that are less fortunate? It is very likely the goodwill we have seen in recent disasters will evaporate as soon as the pandemic moves beyond its first borders. During the Black Death, the pope's doctor commented that 'charity was dead' as those who escaped the first wave did everything they could to defend themselves - priests refused confessions, wives ran from children and husbands. So what chance is there that Hong Kong - with its stockpile of anti-viral drugs, superb medical facilities and a crisis plan polished during the Sars outbreak - will reach out to poorer neighbours with only a fraction of the budget to spend? They may well not have time. If a pandemic takes hold, the scene could be apocalyptic. But why the fear now, when H5N1 first broke in Hong Kong in 1997, infecting 18 and killing six? The reason is as much due to the location of the virus' first outbreak. When new influenza emerges from south China the world knows now to take notice because all previous pandemics have emerged from the region - perhaps even the Black Death. It started in the August, with the first-known casualty a three-year-old boy who played with chickens at his kindergarten. Officials acted quickly and, in what many at the time considered an overreaction, all poultry - 1.5 million birds - were slaughtered after tests discovered about 10 per cent carried the virus. Bird flu just seemed to vanish with the slaughtered animals. There were many questions left unanswered. How did this vicious killer - which could strike down the fit and healthy as easily as the young and infirm - jump species into humans? But it was still there, lurking. The next outbreak, but only in birds, came in South Korea in December 2003, but was largely overlooked as the mop-up after Sars continued. Scientists continued to warn that bird flu was potentially more deadly. In January last year, H5N1 started to really show its pandemic potential. In Thailand, there were 18 cases and 13 deaths. In Vietnam, there were 91 cases, leaving 41 dead. The next month there was a small outbreak in Indonesia that killed three. The virus has now spread into Europe, hitching a ride in a less virulent form with migratory waterbirds, which contracted it from the droppings of domestic birds in lakes and waterways. There have been outbreaks in Russia, Kazakhstan and Inner Mongolia, and this year in Turkey, Romania, Croatia, Macedonia and Greece. By next year, it may have spread further. That wild birds were responsible for spreading the disease was only really proved earlier this year, when Hong Kong virologist Guan Yi published a report in the journal Nature detailing the mass death of birds in Qinghai Lake in the remote northwest of China. The lake was a breeding ground for almost 200 different bird species in the spring before they left for winter grounds around the globe. Dr Yi had to organise contacts on the mainland, secretly collecting dead birds to test for H5N1. Science knew that they would spread the virus, and sure enough, it has landed in Europe this year. The WHO is also worried about Africa, the least likely of any continent to be able to deal with a severe H5N1 outbreak and with few, if any, checks and balances that would alert the international community. Imagine a remote villager becoming sick and dying. Human to human transmission could emerge and pass into the community before the world's agencies had any inkling. But that is unlikely. The scenario the planet is still banking on is that human to human transmission will emerge in southern China and spread from there. All the scientists know is it is likely to happen. We must wait and prepare, hoping governments have stockpiled enough drugs and prepared to ensure any threat is challenged and damage limited.