The Collaborative International Dictionary of English v.0.48:

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.
   [1913 Webster]

WordNet (r) 3.0 (2006):

diathesis
    n 1: constitutional predisposition to a particular disease or
         abnormality

Moby Thesaurus II by Grady Ward, 1.0:

132 Moby Thesaurus words for "diathesis":
   Altmann theory, DNA, De Vries theory, Galtonian theory,
   Mendelianism, Mendelism, RNA, Verworn theory, Weismann theory,
   Weismannism, Wiesner theory, a thing for, affinity, allele,
   allelomorph, animus, aptitude, aptness, bent, bias, birth,
   body-build, brand, cast, character, characteristic,
   characteristics, chromatid, chromatin, chromosome, complexion,
   composition, conatus, conduciveness, constituents, constitution,
   crasis, delight, determinant, determiner, dharma, disposition,
   eagerness, eccentricity, endowment, ethos, eugenics, factor,
   feeling for, fiber, frame, gene, genesiology, genetic code,
   genetics, genius, grain, habit, hereditability, heredity,
   heritability, heritage, hue, humor, humors, idiosyncrasy, ilk,
   inborn capacity, inclination, individualism, inheritability,
   inheritance, kidney, kind, leaning, liability, liking, make,
   makeup, matrocliny, mental set, mettle, mind, mind-set, mold,
   nature, patrocliny, penchant, pharmacogenetics, physique,
   predilection, predisposition, preference, prejudice, probability,
   proclivity, proneness, propensity, property, quality, readiness,
   recessive character, replication, sensitivity to, set, slant,
   soft spot, somatotype, sort, spirit, stamp, strain, streak, stripe,
   suchness, susceptibility, system, temper, temperament, tendency,
   tenor, tone, tropism, turn, turn of mind, twist, type, vein, warp,
   way, weakness, willingness