Although more than 150 people have contracted the H5N1 virus, experts point out that cross-infection to humans is still relatively rare, and usually occurs where people have been in close contact with infected birds.Many scientists fear it may be carried by migrating birds to Europe and Africa but say it is hard to prove a direct link.
But as H5N1 spreads west from its original hotspot of south-east Asia, there is mounting concern that it may combine with a human strain to produce a mutation that is more dangerous and difficult to combat.
