Final answer:
The important discovery is likely referring to genetic disorders, which are pivotal to modern biological theories of abnormality. The Germ Theory of disease and Biochemical Theories of Brain Diseases have advanced our understanding of illness, recognizing a blend of genetic and environmental factors.
Step-by-step explanation:
The disease that has been characterized as one of the single most important discoveries underpinning modern biological theories of abnormality is likely a genetic disorder, a disease, syndrome, or other abnormal condition caused by a mutation in one or more genes or by a chromosomal alteration.
From these theories, we recognize a complex interplay between genetic predisposition (nature) and environmental factors (nurture), contributing to the array of nervous system disorders. Despite tremendous progress, many disorders still remain only partially understood, with treatments often aimed at alleviating symptoms rather than curing the underlying causes.