BBC News Online looks at plague and how it could be used as a weapon.
Scenes during a plague scare in India
What is it?: A disease caused by the Yersinia pestis bacterium (see image A).
Effects and symptoms: There are three types of plague: bubonic, pneumonic and septicemic.
The incubation period for bubonic plague is 2-10 days. Initially a high fever is accompanied by tender lymph nodes - known as buboes. Seizures can also occur. This disease can not be passed on from person to person.
Symptoms of pneumonic plague occur 2-3 days after infection. Early symptoms include fever, coughs, shortness of breath, chills, headache and bloody sputum. Pneumonia develops, and if untreated the mortality rate is 100%. It can be transmitted from person to person.
Septicemic plague is usually a result of complications in the other two versions of plague. Patients have fever, chills, abdominal pain, shock, and bleeding into skin and other organs. Septicemic plague does not spread from person to person.
Use as a weapon: Of the three types of plague the highly lethal pneumonic version would be the obvious choice as a weapon. Bubonic plague may have been used by the Japanese in China during World War II.
The Russians experimented with using it as a weapon, which would have been spread in aerosol form. In the 1950s and 1960s the Americans did likewise. The organism does not have a long life out of the laboratory and can be killed by exposure to sunlight.
In 1970 the UN estimated 50 kilograms of plague sprayed over a large city by plane could result in 150,000 cases of the disease, and about 36,000 deaths.
History: Plague is a disease found naturally in rodents. It first killed humans in massive numbers in the 5th Century. Some estimates say that the second massive outbreak of plague in the 14th Century killed around a third of the population of Europe. The fear the disease still creates was made plain in India when a small outbreak in 1994 caused widespread panic. Over a 1,000 cases are reported annually worldwide.
Production: The disease is naturally occurring across much of the world, including Asia and America. The difficultly in using it as a weapon would be for attackers to find of means of spreading large enough quantities to create an epidemic.
Protection: Vaccines for plague exist, but their effectiveness is difficult to gauge. It is hard to test the environment for the presence of plague and provide early warnings for civilians.
Treatment: Antibiotics are effective against plague, but results depend on identifying and treating patients as soon as possible. People who have been exposed to pneumonic plague victims should consider starting a course of antibiotics.