Psychologists, as a discipline focused on the measurement of biological responses to macroscopic environmental changes (social competition for food, environmental complexity in the quest for food, and social cooperative interaction are some examples), tend to work more with patients who already have some level of intactness in their central nervous systems.
This is not fully true, however. In many cases, even the smallest child or infant responds to emotional cues, even if their capacity to discern the features of their environment is limited.
Psychology often undertakes to assist in human tasks such as learning. In this latter category, we find educational psychologists. And, of course, cognitive behavorial therapists, behavioral therapists, and counseling psychologists often work with mental structures that are not simplifiable ( to the extent that they cannot be represented with robust mathematical models of neurophysiology.)
For this reason, for patients whose illness is both in part underpinned by complex mental processing, and whose illness cannot be fully treated by medications, if we are expecting the illness to be a consequence of illness within mental structures (the psyche), we then may more accurately refer to these patients as having psychologic illness. This does not exclude neurophysiologic illness, but instead implies that the two are entangled in such a way that psychiatric diagnosis can be better refined by a broader model that may be more difficult to represent numerically.
Many of these patients, at one time, were treaated by either psychoanalysts or psychodynamic psychotherapists, but now also may be treated by psychologists emplying the methods outlined above.
The term psychologic illness does not mean that only those with adequate communication capacity can have these forms of illness, but rather that psychologic illness is more difficult to diagnose and treat for patients who are non verbal or only partially verbal.