There is an excellent article at the Asia Times on why H5N1 (bird flu) has government and health organizations so concerned.

Since the strain was first identified in Hong Kong in 1997, over 100 million birds have died or been culled.
60 people have died.

What medical experts have been concerned about is cross-species transmission.

It is thought that a few cases of person-to-person spread of H5N1 viruses have occurred. For example, one instance of probable person-to-person transmission associated with close contact between an ill child and her mother is thought to have occurred in Thailand in September 2004. More recently, possible person-to-person transmission of H5N1 viruses is being investigated in several clusters of human cases in Vietnam.

China may have learned lessons from SARS and is beginning to report bird flu outbreaks. Humans have no immunity to this strain of influenza or any of the mutations.