For the purposes of this Act, an injury sustained, or a disease contracted, by a person is a service injury or a service disease if:

(a) the injury or disease:
(i) was sustained or contracted while the person was a member rendering defence service, but did not arise out of that service; or
(ii) was sustained or contracted before the commencement of a period of defence service rendered by the person while a member, but not while the person was rendering defence service; and
(b) in the opinion of the Commission, a sign or symptom of the injury or disease was contributed to in a material degree by, or was aggravated by, any defence service rendered by the person while a member after he or she sustained the injury or contracted the disease.