Origin: Germanic Old English suffix
Illness has 2 different meanings across 1 category:
impairment of normal physiological function affecting part or all of an organism
"The doctor explained that his sudden fever was a symptom of an underlying illness causing systemic impairment of his immune response."
An instance of a disease or poor health.
"The doctor asked me to describe my illness so he could determine whether it was contagious."
In plain English: An illness is when your body feels sick and isn't working the way it should.
"The doctor asked about any recent illness before prescribing the medicine."
Usage: Use "illness" to refer to a specific episode of sickness or a prolonged period of poor health affecting an individual. Avoid using it to describe the general condition of being sick in favor of simpler terms like "sickness" or "disease."
The word illness is formed by adding the suffix -ness to the adjective ill. It entered English as a straightforward combination of these two existing elements to describe the state of being unwell.