Source : Webster's Revised Unabridged Dictionary (1913)

Diathesis \Di*ath"e*sis\, n. [NL., fr. Gr. ?, fr. ? to place
   separately, arrange; dia` through, asunder + ? to place,
   put.] (Med.)
   Bodily condition or constitution, esp. a morbid habit which
   predisposes to a particular disease, or class of diseases.

Source : WordNet®

diathesis
     n : constitutional predisposition to a particular disease or
         abnormality
     [also: {diathses} (pl)]