Disease
Etymology
From Old French "desaise," meaning lack of ease or discomfort — "des-" (away, without) plus "aise" (ease). It began as a broad word for any distress or trouble and only later hardened into the biomedical sense of a named, mechanistic disorder.
noun · /dih-ZEEZ/
A specific bodily or mental disorder with identifiable causes, mechanisms, and signs.
What it names
Use it for the underlying pathology itself, distinct from how it feels to live with it or how society treats the sufferer.
Example
He had the disease for years before any illness — any felt suffering — appeared.
Related idea
Medical sociology splits three words people blur: disease is the biological pathology, illness is the subjective experience of being unwell, and sickness is the social role — being excused from work, treated as a patient. You can have one without the others.