What illness did Hezekiah recover from?

Answer: A boil

Hezekiah was mortally ill; at Isaiah's word a poultice (KJV: lump) of figs was laid on his boil, and he recovered.

“Then Isaiah said, “Prepare a poultice of figs.” So they brought it and applied it to the boil, and Hezekiah recovered.”— 2 Kings 20:7 (BSB)