They are easily available.
Biological pathogens can be obtained from nature, hospital laboratories, university research facilities, etc.
They can be hard to detect.
Small quantities can have potentially deadly or incapacitating effects on a susceptible population.
They can be used covertly.
They can be spread throughout large areas by natural convection, air or water currents.
They can be easily spread.
Ventilation systems in buildings is one way biological agents may be spread. In addition, transportation facilities could become part of the dissemination system by carrying biological agents far from their initial source.