Patients with severe H5N1 disease have unusually higher serum concentrations of proinflammatory cytokines and chemokines. Levels of plasma macrophage attractant chemokines CXCL10 (IP-10), CXCL9 (MIG), and CCL-2 (monocyte chemoattractant protein 1, MCP-1) and of neutrophil attractant interleukin-8 (IL- 8) were substantially higher in patients with H5N1 disease compared with those experiencing seasonal influenza virus and were significantly higher in H5N1 patients who died compared with those who recovered [23, 24] . The elevation of plasma cytokine levels was positively correlated with pharyngeal viral load [23] and may simply reflect more extensive viral replication, and consequently, direct viral pathology rather than being causative of the pathology observed in H5N1-infected patients. Compared with human H1N1 and H3N2 influenza viruses, infection of human primary macrophage cultures in vitro with H5N1 viruses also lead to the hyper-induction of proinflammatory cytokines [25] .