Illness geht eher in den Bereich "Krankheit, bei der man sich schlecht fühlt", "an der man leidet", "wegen der man fehlt".
Diseases können Wehwehchen aller Art bis zu schweren Krankheiten sein. Ich glaube dieser Begriff ist umfassender als Illness. Bin aber leider kein "native".

as far as I can tell, disease is actually more a chronical thing such as a handicap or sth like that, whereas an illness can also be a very bad cold...But I´m neither a native nor a doctor.

Medically, really no difference in definition - patients tend to talk about their illnesses and doctors more about diseases. Some conditions are traditionally described as "disease" and others as "illness" and would sound wrong with the other term. Examples that spring to mind are "a flu-like illness", "autoimmune disease", "psychosomatic illness", "Alzheimer's disease".

Dorland's Illustrated Medical Dictionary:
"disease (dis·ease) [Fr. dès from + aise ease] - any deviation from or interruption of the normal structure or function of a part, organ, or system of the body as manifested by characteristic symptoms and signs; the etiology, pathology, and prognosis may be known or unknown."
http://www.mercksource.com/pp/us/cns/cns_hl_d...
"illness (ill·ness) - disease"
http://www.mercksource.com/pp/us/cns/cns_hl_d...
another term sometimes used with the same meaning is "sickness"
"sickness (sick·ness) - disease"
ttp://www.mercksource.com/pp/us/cns/cns_hl_dorlands.jspzQzpgzEzzSzppdocszSzuszSzcommonzSzdorlandszSzdorlandzSzdmd_s_11zPzhtm#12734896
(e.g. "radiation sickness", "altitude sickness", "sleeping sickness")
HTH

Vielen Dank.

Hier spielt auch die Problematik der Adjektive "ill/sick" mit hinein.
Das Longman Dictionary of Contemporary English meint hierzu:

"In British English the word "ill" means not healthy. Americans usually use "sick" for this meaning. In British English "sick" can be used in this way, but is more commonly used in expressions such as "be sick" for "feel sick" meaning to "vomit" or feel that you are going to "vomit"."

Ich erinnere mich in diesem Zusammenhang an den Satz:

The dog was sick all over the carpet. (Der Hund hat den ganzen Teppich vollgekotzt).