A recent report assembled at the behest of Congress concludes that a biological terror attack is quite likely by 2013. This conclusion seems quite reasonable.
Biological weapons have been used for centuries. Early forms were fairly crude: dumping dead animals in wells, flinging diseased corpses over city walls during sieges, and similar such activities. Later on, the art of biological warfare was refined a bit. One excellent example is the use of blankets infected with small pox as weapons against the indigenous people of America. During the World Wars and Cold War, biological warfare was further enhanced as natural diseases were intentionally enhanced and new strains were created. This research continues to this day; mostly under the guise of developing defenses against biological weapons.
It is easy enough to imagine how terrorists could gain access to biological weapons. Lax security, bribery, and so forth can allow them access to military grade weapons. Naturally, terrorist groups that are supported by states could simply be given such weapons.
Terrorists could also create their own. While they will typically lack the facilities of a nation, cultivating deadly disease agents is relatively easy to do. They are readily available (after all, people are naturally infected) and usually the challenge is to keep them from spreading rather than spreading them. While the terrorists will most likely lack the ability to create weapons on par with those made by nations, the naturally occurring diseases are often quite dangerous enough.
The main challenge that the terrorists face are infecting a population in an effective manner and getting the disease to spread enough to do serious damage. Of course, even if they were only able to affect a limited population, this would certainly help to create significant terror. After all, “conventional” terrorists attacks tend to kill relatively few people and create relatively little destruction compared with something like an actual battle. As such, the terrorists do not need to duplicate the military grade biological weapons of mass destruction or even the battlefield versions. They just need a biological weapon that can infect a relatively small number of people. Of course, if they use a natural disease and only infect a small number of people, there will be doubts as to whether or not such an attack is really an attack or not. After all, if a terrorist group claimed credit for the winter flu season, then they would obviously not be taken seriously.
So far terrorist groups have not employed such weapons. One reason might be that they lack access to an effective biological weapon that would serve their purposes (that is, one that would be clearly recognized as a weapon and not a natural breakout). Another reason might be that the biological weapon threshold is one that even the terrorists are reluctant to cross. A third reason is that there are practical concerns about such weapons that are holding terrorists back. For example, there might be greater backlash against the deployment of such a weapon. After all, biological weapons tend to be regarded as being far worse than conventional weapons. As another example, such weapons tend to be too indiscriminate and could spread too far-even for the purposes of terrorism. There are probably other reasons as well.
If the terrorist are able to get past or around these obstacles, then biological weapons will probably be used. For example, the history of terrorism is a history of crossing ever more evil thresholds. So, it is probably just a matter of time before terrorists cross the biological threshold in a large scale attack.