Relapse
Etymology
From Latin "relapsus," past participle of "relabi" (to slip or slide back), from "re-" (back) and "labi" (to slip, fall). The image is of sliding backward down a slope you had begun to climb.
noun · /REE-laps/
The return of a disease or its symptoms after a period of improvement.
What it names
Use it for backsliding while recovery was still underway — the illness resurfacing before it was ever fully gone.
Example
She had a relapse three weeks into treatment, and the fever returned worse than before.
Related idea
A relapse is the disease coming back before you fully recovered, while a recurrence is its return after a genuine gap of full health or remission. The distinction matters in addiction and cancer, where "relapse" implies the underlying condition never truly resolved.