Smallpox has a number of unique characteristics which made its eradication possible. Its symptoms develop quickly, making those infected aware of the disease at an early stage and reducing the possibility of them unknowingly transmitting the disease to others. Because it is almost completely specific to humans, there is a very low possibility of smallpox being kept alive in animals to reinfect humans. The availability of effective vaccines was also a necessary factor. Finally. The high level of mortality from the disease made it easier to achieve global agreement on its eradication.