What is the primary factor that defines a Category A Infectious Substance?

The primary factor that defines a Category A Infectious Substance is indeed the risk of infection it poses. These substances are known to contain pathogens that can lead to serious disease or death in humans or animals when they are exposed. The categorization process is based on the inherent characteristics of the substances themselves, particularly their ability to cause infections effectively.

In the context of transportation regulations, these substances require strict handling and packaging procedures to mitigate the risk of exposure during transit. While factors such as volume, packaging, and geographic location are relevant in the overall handling process and in determining certain logistics, they do not fundamentally define what makes a substance fall into Category A. Thus, it is the infectious risk that is critical in identifying and managing these dangerous goods appropriately.