[wtallis]

"Exposure time" usually implies you're spending some period of time in proximity to an effectively unlimited reservoir of the pathogen (eg. an infected person coughing frequently) and your exposure is at some level of concentration that depends on the type/route of exposure, so that the total amount of pathogen that ends up in your body to challenge your immune system is proportional to the duration of time you spend being continually exposed.

A phrase like "the raw quantity of infectious agent and the exposure rate" is calling attention to the fact that one brief intense exposure vs repeated small exposures over a long time span may have vastly different outcomes despite presenting your immune system with the same number of microbes to deal with.