BBC News Online looks at smallpox and how the deadly, infectious disease could be used as a weapon.
A reconstruction of the effects of smallpox on skin
What is it?: Smallpox is a disease caused by the variola virus (see image A).
Effects and symptoms: Smallpox kills about a third of those infected. Symptoms include fever, tiredness and headaches, followed by a rash and deep lesions. These occur mostly on the face or on the extremities and are filled with pus. As time passes these scab over, and if the infected person recovers the scabs will eventually fall off, but can leave scarring.
Smallpox's 12-day incubation period means it can spread before it is detected. It is contracted by breathing the virus in, like the common cold, or from infected articles such as bedding. There is no effective treatment.
Use as a weapon: The virus could be spread by being sprayed over populations, or suicide attackers could infect themselves to pass the virus on to others. Either would trigger an outbreak which would then spread if not contained.
History: Smallpox is believed to have killed more people in human history than almost any other disease, but it was finally wiped out in the 1970s. It has been used in germ warfare, by the British while fighting in America against indigenous tribes in the 18th Century.
Inoculation against the disease began at the turn of the 18th Century. After a campaign by the World Health Organisation it was eradicated in 1977. Vaccinations stopped in 1980.
Production: Smallpox no longer occurs naturally but two approved labs in the USA and Russia have stocks of the virus. The smallpox vaccine cannot be used to give people smallpox or to produce the full-blown virus, so it could not be used to make weapons.
Protection: There is a smallpox vaccine, but it does not offer a 100% guarantee of safety. It also carries a low but significant chance of severe side effects. The US Government is planning to make vaccines available to all its citizens and mandatory to soldiers. The British Government also has plans to vaccinate key medical and military teams.