a strong person unexpectedly falling ill
Proverbial expression: even a robust person can suddenly get sick. Often used when someone who rarely gets ill catches a cold or similar minor ailment. The literal image is 'a demon getting sunstroke'.
Him taking a day off with a cold? That's a case of the devil getting sunstroke.
The always energetic department head was hospitalized. Truly a case of a strong person unexpectedly falling ill.