For my bones are filled with a loathsome disease - This would seem to indicate the seat of the disease, though not its nature. The word used here, according to Gesenius (Lexicon), properly denotes the internal muscles of the loins near the kidneys, to which the fat adheres. The word rendered "loathsome" - the word "disease" being supplied by our translators - is derived from קלה qâlâh, a word which means to roast, to parch, as fruit, grain, etc.; and then, in the form used here, it means scorched, burned; hence, a burning or inflammation; and the whole phrase would be synonymous with "an inflammation of the kidneys." The word used here does not imply that there was any eruption, or ulcer, though it would seem from Psalm 38:5 that this was the fact, and that the inflammation had produced this effect.
And there is no soundness in my flesh - See Psalm 38:3. His disease was so deep-seated and so pervading, that there did not seem to be "any" soundness in his flesh. His whole body seemed to be diseased.

For my loins are filled with a loathsome disease - Or rather, a burning; נקלה nikleh, from קלה kalah, to fry, scorch, etc., hence נקלה nikleh, a burning, or strongly feverish disease.
There is no soundness in my flesh - All without and all within bears evidence that the whole of my solids and fluids are corrupt.

For my loins are filled with a loathsome disease,.... The word here used has the signification of burning (k); and the Targum renders it, "my loins are filled with burning"; a burning fever was upon him, or there was an inflammation in those parts; a hot burning ulcer, which might be nauseous; and so was true in both senses. Aben Ezra interprets it abominable and vile; something not fit to be mentioned; and so Kimchi and Ben Melech. The word is rendered sometimes "lightly esteemed"; as in 1-Samuel 18:23; and Jarchi thinks it has this sense here; and the meaning is, that he was vile in his own eyes, and mean in his own esteem. Doubtless the psalmist has reference to something more than a bodily disease; at least not to that only, but to the disease of his soul also, sin, which has the nature of a disease; it is an hereditary one, which is derived from one to another by propagation; it is universal, and reaches to all men, and to all the parts of the body and powers of the soul; it is a complication of disorders: it is in its own nature mortal, and ever incurable but by Christ; and, as here, it is a loathsome one; it is loathsome to God, and to all sensible sinners: and when the psalmist says his loins were filled with it, it may signify that it was an internal disorder that was in him; sin that dwelt in him, a law in his members; and may denote the aboundings of sin in him, the swarms of corruptions that were in him; as also the pain it gave him, and the quick sense he had of it;
and there is no soundness in my flesh: which is repeated, see Psalm 38:3; partly for confirmation's sake, and partly to show the continued sense of it, as persons under a disorder are continually making mention of it.
(k) R. Joseph Kimchi & Abendana "ardore", Pagninus, Vatablus; "ardens ulcus", Musculus, so some in Vatablus; "tostione", Piscator; "adustione", Gejerus; so the Targum; "adusto", Gussetius, Ebr. Comment. p. 742.

Disease - The disease might be some burning fever, breaking forth outwardly in carbuncles, or boils. It is true, this and the other expressions may be taken figuratively, but we should not forsake the literal sense of the words without necessity.