Operationally significant biological incidents are caused by pathogens of various types; pathogens may be viral, bacterial, or parasitic. Each type of pathogen poses specific response and recovery challenges. The challenge is distinguishing a large-scale biological event from a normally occurring disease fluctuation (e.g., seasonal influenza, or norovirus outbreak on cruise ships). The threat of a pandemic has serious national security implications for the United States. Because humans may have little or no immunity to a new or modified pathogen, pandemics may occur with substantially higher infection or mortality rates.