Quantity of toxin release by pathogen in host
As I commented, a bolus is a single (often large) dose of a drug or compound given (released) all at once. Please read the very next sentence after the one you quoted: 


  For example, the lethal and edema toxins of Bacillus anthracis disseminate within the host bloodstream and act upon both immune and nonimmune cells.


This gives an example of a pathogen releasing multiple toxins, although there are other pathogens such as Clostridium tetani which release a bolus that consists of only one toxin. So, it can be one or the other.