Hard to stop literal mind occasionally showing disease (7)
I believe the answer is:
typhoid
'disease' is the definition.
(typhoid is a kind of disease)
'hard to stop literal mind occasionally' is the wordplay.
'hard' becomes 'h' (abbreviation used in pencil classifications).
'to stop' is an insertion indicator (inserted letters plug or stop a gap).
'literal' becomes 'typo' (synonyms).
'occasionally' means one should take alternating letters (letters taken at regular intervals or occasions).
The alternating letters of 'mind' are 'id'.
'h' inserted into 'typo' is 'typho'.
'typho'+'id'='TYPHOID'
'showing' acts as a link.
(Other definitions for typhoid that I've seen before include "Highly infectious bacterial fever" , "Infectious intestinal disease" .)