Case definition, in epidemiology, set of criteria used in making a decision as to whether an individual has a disease or health event of interest. Establishing a case definition is an imperative step in quantifying the magnitude of disease in a population. Case definitions are used in ongoing public health surveillance to track the occurrence and distribution of disease within a given area, as well as during outbreak investigations in field epidemiology.
This is the "case" that's being referred to. Increase in cases, for this discussion (and in the past) is always about reported cases. This doesn't mean reported to the CDC per se, but diagnosed.
Population has X amount of infection. Testing reveals those. Each one becomes a case as the testing increases.