Avian influenza viruses are widespread in birds, contagious in humans, and are categorized as low pathogenicity avian influenza or highly pathogenic avian influenza. Ferrets are susceptible to infection with avian and human influenza A and B viruses and have been widely used as a model to study pathogenicity and vaccine efficacy. In this report, the natural history of the H5N1 influenza virus A/Vietnam/1203/04 influenza infection in ferrets was examined to determine clinical and laboratory parameters that may indicate (1) the onset of disease and (2) survival. In all, twenty of 24 animals infected with 7 × 10(5) TCID(50) of A/Vietnam/1203/04 succumbed. A statistical analysis identified a combination of parameters including weight loss, nasal wash TCID(50), eosinophils, and liver enzymes such as alanine amino transferase that might possibly serve as indicators of both disease onset and challenge survival.