Any of the susceptible birds can become infected with bird influenza virus when they have contact with contaminated nasal, respiratory, or fecal material from infected birds. Fecal-to-oral transmission is the most ordinary mode of spread between birds.

The spread of a lethal strain of bird flu in these past two years has sparked fears of a new pandemic. Although more than 200 people have contracted and are affected with the H5N1 virus, experts point out that cross-infection to humans is still relatively rare, and usually take place where people have been in close contact with infected birds.

Many scientists fear migrating birds to Europe and Africa might carry it but say it is hard to prove a direct link. Others say that farming methods and human lifestyle could be accelerating the spread.

Spread of bird flu in many countries

But as H5N1 continues its journey through Europe and Africa from its original hotspot of south-east Asia, there is mounting concern that it might combine with a human strain to produce a mutation that is more dangerous and difficult to combat.

Of the few bird influenza viruses that have crossed the species barrier to infect humans, H5N1 has lead to the largest number of detected cases of severe disease and death in humans. In the current outbursts in Asia and Europe more than half of those infected with the virus have died. Most cases have taken place in previously healthy children and young adults.

However, it is probable that the only cases currently being reported are those in the most severely ill people, and that the full range of illness caused by the H5N1 virus has not yet been defined.

Spread of virus among the humans

So far, the spread of H5N1 virus from person to person has been restricted and has not continued beyond one person. Nonetheless, because all influenza viruses have the capability to change, scientists are concerned that H5N1 virus one day could be able to infect humans and spread easily from one person to another.

Because these viruses do not generally infect humans, there is little or no immune protection against them in the human population. No one can predict when a pandemic might take place. However, experts from around the world are watching the H5N1 situation in Asia and Europe very closely and are preparing for the possibility that the virus might begin to spread more easily and widely from person to person.