Rashi: They are afflicted with illness due to their sins.

Radak: Only here it says Evilim, for captivity is at once, but illness begins light, and gets worse. They are fools for not realizing that it is like a messenger sent to them, that they should repent. They do not sense this, like a fool who does not sense evil that comes to him due to his foolish deeds. If they were wise, they would search their ways and fix them before the illness worsened. Fools do not realize until they are very afflicted and despise all food, like Elihu said "v'Huchach b'Mach'ov Al Mishkavo v'Rov Atzamav Esan; v'Zihamatu Chayaso Lechem" (Iyov 33:19-20).

Malbim: An Evil is unsure. He does not believe that the danger he was in and his salvation were all from Hashem. He attributes everything to chance. This is (a) "mi'Derech Pish'am" - they are used to sin, to the point that their hearts are not prepared to accept Musar, or (b) "me'Avonoseihem" - crooked intellect and heresy. Hashem afflicts them and humbles them a third time, via illness.