Pathogens such as bacteria and viruses, are living, reproducing microorganisms that infect the body and cause disease and death. They may be contagious or non-contagious. Examples of pathogens that have been used as biological weapons are smallpox, anthrax and plague.
Examples are botulinum toxin and ricin.
To be effective as weapons, both pathogens and toxins need to be delivered intact to their target, in such a way so as to be able to enter the body in sufficient amounts to cause infection or intoxication. The modern biological weapon tends to be a fine powder, containing dried or otherwise stabilised particles of pathogen, which disperses quickly and invisibly in the air over a wide area. It may be sprayed from a plane to drift over a wide area with the wind, spread in a certain pattern by specially designed bomblets, or introduced into confined spaces. Huge numbers of people can be killed by a small amount of substance. A 1993 study estimated that a warhead filled with 30 kg of anthrax used against a large city would kill between 30,000 and 100,000 people over an area of 10 sq km.