Variola virus is considered as an ideal bioterrorist weapon for the following reasons:
It is highly transmissible by the aerosol route from infected to susceptible persons.
The civilian populations of most countries contain a high proportion of susceptible persons.
Smallpox is associated with high morbidity and about 30% mortality.
Initially, diagnosis of a disease that has not been seen for 20 years would be difficult.
At present, other than the vaccine, which may be effective in the first few days post-infection, there is no proven drug treatment available for clinical smallpox.
Mahy, Antiviral Res. 57, 1-5 (2003)