The Compendium of Caraka 1.10.10

1.10.10

sādhyānāṃ trividhaścālpamadhyamotkṛṣṭatāṃ prati /
vikalpo na tvasādhyānāṃ niyatānāṃ vikalpanā //

For curable diseases there is a threefold classification according to mild, moderate, and severe degrees;
but there is no such classification for incurable diseases, which are fixed.