John Calvin Commentary Deuteronomy 28:35

"Jehovah will smite thee in the knees, and in the legs, with a sore boil, whereof thou canst not be healed, from the sole of thy foot unto the crown of thy head." — Deuteronomy 28:35 (ASV)

The Lord shall smite thee in the knees. Since death is common to the whole human race, they must also, by necessity, all be subject to disease; nor is it a matter of surprise that the whole posterity of Adam, which is infected with the taint of sin, should thus be liable to many afflictions, which are the wages of sin.

But, since the offenses of all are not alike, God also maintains a just proportion in the execution of His various punishments; thus, in this passage He does not speak only of common maladies, but of those by which He openly shows His vengeance against the transgressors of the Law, such as incurable diseases.