Figure 3. The host immune response to an infection influences the disease outcome.
Infection with H5N1 influenza virus can cause very different disease outcomes in different reservoir and spillover host species. Waterfowl, such as wild ducks, are the natural host for this virus and develop a limited inflammatory response that is associated with low levels of cytokine expression. Intermediate hosts, including mice, pigs and ferrets, are often used to study this infection and display mild to severe disease symptoms (depending on the H5N1 virus strain used) that are associated with increased levels of pro-inflammatory cytokines. By contrast, spillover hosts such as chickens and humans display a rapid and strong inflammatory response, often referred to as hypercytokinaemia (or cytokine storm) and the infection becomes systemic, causing severe disease symptoms and high mortality rates.