Unlike the tools of chemical terrorism , biological weapons were not designed - and cannot ordinarily be used for - pinpoint attacks; their principle purpose is mass devastation. The results of a biological attack are not immediate: they become apparent several hours or days later (after some of the victims have left the site of the attack). This may make it harder to detect the perpetrator and target of the attack.
Biological weapons are not as common, accessible, or available as chemical weapons. For a terrorist organization, the two main sources of biological weapons are home manufacture or purchase from sovereign states.
Homemade biological weapons require the use of sophisticated biological laboratories and resources of diverse kinds, which, on the whole, are not available to terrorist organizations. However various countries (particularly those that are unable to obtain nuclear weapons) have stockpiles of biological and chemical weapons. Under certain circumstances, some of them may deliver biological weapons to terrorists that enjoy their patronage.
Once a terrorist organization has succeeded in obtaining biological weapons, it can move them from place to place very easily (as with chemical weapons) without fear of detection. Similarly, the weapon can be transported to the vicinity of the target and, if necessary, concealed there until used.