The causes of the pandemic are still debated. Although the causal agents that produce influenza have been identified, there is a great deal concerning its origins and epidemiology that is still unknown. The key factor cited in explaining the intensity of the outbreak is the virulence of the virus strain involved. Some theories suggest that while the virus was present before 1918 it developed its greater virulence through mutation. Other theories postulate animal reservoirs of influenza, where the disease lurks between epidemics. The reason for the deadliness of Spanish influenza clearly lies in the balance of two factors, each of which defines the other: the virulence of the virus and the vulnerability of its victims. But the influenza of 1918-1919 still keeps its final secrets — where did the flu go? Could it return?