The diseases of the soul...

Foucault, The Care of the Self, p.58:

And the establishment of the relationship to oneself as a sick individual is all the more necessary because the diseases of the soul - unlike those of the body - do not announce themselves by the suffering that one perceives; not only can they go undetected for a long time, but they blind those whom they afflict...

The insidious thing about the diseases of the soul is that they pass unnoticed, or even that one can mistake them for virtues (anger for courage, amorous passion for friendship, envy for emulation, cowardice for prudence).