。
Her illness will be cured soon.
Literal
[topic-は] before-long illness [subject-が] will-heal probably.
治る ('heal, be cured') is the intransitive verb — illness or wound heals on its own. The transitive twin is 治す ('cure, heal [something]'). が marks the subject 病気 ('illness') of the intransitive predicate. Note the 'double subject' surface here: 彼女は (topic) and 病気が (subject) — Japanese handles body-part and personal-property predicates this way (花子は鼻が高い 'Hanako has a high nose'), with one entity holding the topic frame and another marking the actual subject of the predicate. でしょう adds polite speculation.