The recent spread of the most dangerous H5N1 bird flu virus has sparked a race to find an effective drug to fight it. Paradoxically, scientists say its arrival in Turkey shows it could have weakened, and in so doing become more of a threat.Birds that catch it are not dying straight away, meaning they have time to cross continents with the virus. What is more, it keeps evolving, which makes the search for beating it even harder. Just recently a strain has been discovered that resists the stockpiled anti-viral Tamiflu drugs. The World Health Organisation also says a global vaccination programme is out of the question. Spokesman Peter Cordingley said: “For it to stop the spread of a bird flu pandemic, we will have to produce enough vaccine to inoculate every single person in the world, so that’s six billion plus units of vaccine. Quite clearly that cannot be done.” For the moment there have been no confirmed cases of the disease being transferred from human to human. It remains rare – fewer than 100 cases worldwide – for the virus to transfer from farmed birds to humans. In general H5N1 is being carried in the intestines of wild birds. But it does have the ability to jump the species barrier, meaning it could swap genes with human flu, creating a new, even more harmful virus.