Jamieson, Fausset and Brown on Isaiah 1:5

5. Why--rather, as Vulgate, "On what part." Image from a body covered all over with marks of blows (Psa 38:3). There is no part in which you have not been smitten.

head ... sick, &c.--not referring, as it is commonly quoted, to their sins, but to the universality of their punishment. However, sin, the moral disease of the head or intellect, and the heart, is doubtless made its own punishment (Pr 1:31; Jr 2:19; Ho 8:11). "Sick," literally, "is in a state of sickness" [Gesenius]; "has passed into sickness" [Maurer].