Six bioagents are commonly classified as class A agents, meaning that they could cause sufficient morbidity and mortality to seriously compromise the function of a city or other large area. These are anthrax, smallpox, plague, botulinum toxin, tularaemia and viral haemorrhagic fever. Aerosol release is the most likely route. It is hard to get enough of a bioagent into the water or food supply to paralyze a city or other large area.
Whilst preparedness for a bioterrorist attack is improved in certain places, notably the USA, most cities do not have even basic responses, such as round-the-clock call for public health and emergency workers to respond immediately to reports or questions of suspicious cases in emergency rooms or elsewhere; interconnection of hospitals, public health systems and laboratories to facilitate rapid detection and management; appropriate training of detection staff; emergency medical committees for for conjoint planning at the local and regional levels; capabilities for mass vaccination or distribution of antibiotics or respirators within a few days.
The specialized knowledge required to manufacture deadly biochemical weapons is relatively accessible as are the necessary materials. Poisonous gases are no more difficult to smuggle or make than drugs.