What is a common characteristic of biological agents used in bioterrorism?

Biological agents used in bioterrorism often possess the capability to be transmitted through various vectors, including air and water, which enhances their potential for widespread impact. This characteristic makes them particularly dangerous, as individuals may be exposed without immediate awareness, leading to rapid dissemination among populations. Such transmission routes can contribute to the difficulty in containment and management of biological threats.

The other options do not accurately describe common traits of biological agents. For instance, not all biological agents are visible to the naked eye, and many are microscopic, underscoring the stealthy nature of such threats. Additionally, biological agents can indeed have both physical and psychological effects, which complicates their impact on public health. Lastly, while some agents may require longer exposure to cause illness, others can produce effects with minimal exposure time, thus varying significantly in their required exposure for effectiveness.