"As to my constitution, it has
always been sound. I have never suffered from any
illness in my life, save fleeting attacks of
cephalalgia, the result of too prolonged a
stimulation of the centres of cerebration. My father
and mother had no sign of any morbid diathesis, but I
will not conceal from you that my grandfather was
afflicted with podagra."

Source: The Collins English Dictionary © 2000 HarperCollins Publishers:

diathesis [daı'æθısıs]
noun
(plural: -ses [-ˌsiːz])
a hereditary or acquired susceptibility of the body to one or more diseases
[ETYMOLOGY: 17th Century: New Latin, from Greek: propensity, from diatithenai to dispose, from dia- + tithenai to place]
diathetic [ˌdaıə'θɛtık] adjective

I have often seen it used for a collection of symptoms for which an exact diagnosis is difficult: e.g. a hemorrhagic diathesis could mean bleeding for which the cause is obscure, perhaps an unstudied genetic disorder.