“Diseases of the mind are more common and more pernicious than diseases of the body.”
Book III, Chapter III
Tusculanae Disputationes – Tusculan Disputations (45 BC)
Original
Morbi perniciosiores pluresque sunt animi quam corporis.
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Marcus Tullius Cicero 180
Roman philosopher and statesman -106–-43 BCRelated quotes

Salon interview (1997)
Context: I'm always astounded at the way we automatically look at what divides and separates us. We never look at what people have in common. If you see it, black and white people, both sides look to see the differences, they don't look at what they have together. Men and women, and old and young, and so on. And this is a disease of the mind, the way I see it. Because in actual fact, men and women have much more in common than they are separated.

Edward Young, "Night Thoughts," (1742-1745) Part IX http://www.litgothic.com/Texts/young_night_thoughts.pdf.
Misattributed

“Disease makes men more physical, it leaves them nothing but body.”
Source: The Magic Mountain (1924), Ch. 4
“Melancholy
Is not, as you conceive, indisposition
Of body, but the mind's disease.”
Act III, sc. i.
The Lover's Melancholy (1628)

Interview in the documentary-film The Game Changers by Louie Psihoyos (2018).
Source: The Veneration of Life: Through the Disease to the Soul (1999), p. 9

“For if vicious propensity is, as it were, a disease of the soul like bodily sickness, even as we account the sick in body by no means deserving of hate, but rather of pity, so, and much more, should they be pitied whose minds are assailed by wickedness, which is more frightful than any sickness.”
Nam si uti corporum languor ita vitiositas quidam est quasi morbus animorum, cum aegros corpore minime dignos odio sed potius miseratione iudicemus, multo magis non insequendi sed miserandi sunt quorum mentes omni languore atrocior urguet improbitas.
Prose IV; line 42; translation by H. R. James
Alternate translation:
For as faintness is a disease of the body, so is vice a sickness of the mind. Wherefore, since we judge those that have corporal infirmities to be rather worthy of compassion than of hatred, much more are they to be pitied, and not abhorred, whose minds are oppressed with wickedness, the greatest malady that may be.
The Consolation of Philosophy · De Consolatione Philosophiae, Book IV

“Beware
At war
Or at peace,
More people die
Of unenlightened self-interest
Than of any other disease”