Psalm 89:20-23:

I have found David my servant; with my holy oil have I anointed him:
With whom my hand shall be established: mine arm also shall strengthen him.
The enemy shall not exact upon him; nor the son of wickedness afflict him.
And I will beat down his foes before his face, and plague them that hate him.

Notes on Psalm 89:20-23:

Here God confirmed something that He had already said in Deuteronomy 7:15 – that He would lay sicknesses on those who hate His people.  As shown in that verse, His covenant was to take away sicknesses from His people, NOT to put evil diseases on them.  If you are not a hater of God’s people, there is no reason for sickness to be on you.  If you ARE a hater of God’s people, under the New Covenant, mercy is available to you and you can therefore be healed of any disease.

See also:

Sickness as Chastening and Judgment in the Old Testament