Pigs have cells in their trachea that allow for both avian and human flu infections. If pigs carry both human and avian strains at a given time replication detail can be traded among the flu species and within the pig host new combinations could arise.

So although the pig variety may be less virulent than its avian-oriented relatives, virologists believe transference to the pig population may be a precursor to human infection.

When H5N1 viruses were isolated from pigs in Indonesia and were tested on mice the pig-oriented variation was found to be much less devastating to the exposed mice than the avian H5N1 species.

In growing in pigs, the virus may have become less harmful to mammals in general but it might also mean the virus is one step closer to turning into a human pandemic strain.

However pig infections are thought to happen only occasionally and it is not clear at present whether the H5N1 virus has truly adapted to pigs.