Some experts have speculated that suicide terrorists might deliberately infect themselves with a contagious disease such as smallpox or plague, and then spread it to others by direct contact. This means of dissemination would be possible, however, only during the very brief time window when the terrorists were contagious but not visibly ill or incapacitated. (Such a time window may not exist for smallpox.)  Moreover, even suicide terrorists willing to die instantly in an explosion might think twice about suffering a slow and painful death from smallpox or plague.

In general, the historical record suggests an inverse relationship between the probability and scale of future bioterrorist attacks: small-scale incidents, such as food contamination or the anthrax mailings, will probably occur and cause limited damage, whereas worst-case events such as a smallpox epidemic are much less likely but would be potentially catastrophic.

The September 11 attacks forced some terrorism analysts to rethink their earlier threat assessments to some extent. Particularly troubling was the systematic way that the 19 Al Qa'ida terrorists planned the operation over a period of five years, learned to fly sophisticated passenger aircraft, and carried out the logistically complex attack. Although Al Qa'ida is interested in acquiring biological weapons, no evidence to date suggests that it has succeeded. Nevertheless, if Al Qa'ida were to apply the same systematic approach to the acquisition, weaponization, and delivery of biological threat agents, it could probably overcome the technical obstacles.

In addition, several factors have increased the ability of terrorist groups to obtain pathogens and other materials needed to prepare and carry out a bioterrorist attack:

• Spread of Relevant Technologies:
  Much of the equipment and materials used to grow microorganisms are "dual use"—with peaceful, commercial as well as military applications—and are available from commercial suppliers. For example, stainless steel fermentation tanks suitable for growing anthrax are routinely used to produce commercial products such as vaccines, vitamins, food supplements, biopesticides, and fermented beverages, complicating attempts to restrict exports. Similarly, the various types of nutrient media ("broth") used to grow microorganisms are widely available.
• Spread of Relevant Know-How:
  Standard laboratory methods for cultivating pathogens such as anthrax bacteria are widely known. In addition, formerly classified information about how to convert biological agents into effective weapons has become more accessible to the public through the routine declassification of government documents and recipes posted on the Internet. Some data about production and dispersal of biological agents that is posted on website and reproduced in so-called terrorist cookbooks is accurate, some is not.
• Increasing Availability of Advanced Technologies:
  Basic genetic engineering techniques are now taught in many colleges and even high school biology labs. These methods could make it easier for sophisticated terrorists to genetically modify microbes, rendering them more deadly or resistant to standard antibiotics.