To pose a significant threat to humans, a novel influenza A virus must be able to replicate in the human respiratory tract and be transmitted efficiently from person to person. This is clearly demonstrated by the recently emerged H5N1 avian influenza virus, which causes rapidly overwhelming pneumonia, but has only rarely spread from a patient to close contacts. Animal models have been developed to evaluate the pandemic potential of influenza viruses, especially for the H5N1 agent. In one recent study, Yen et al. (2007) housed ferrets infected with a neuraminidase inhibitor resistant recombinant A/Vietnam/1203/04 (H5N1) influenza virus together with uninfected ferrets and monitored illness, detectable virus shedding lung viral titers and death. They observed only limited transmission of infection to health contacts, which correlated with low amounts of virus in respiratory secretions. The authors concluded that molecular determinants in addition to a virus's receptor binding affinity were responsible for influenza virus transmission among mammalian species (Yen et al., 2007).