You don't understand the value of health until you get sick.
Literal
Sick became [for-first-time-て初めて] health [possessive-の] value [subject-が] is-understood.
~て初めて links illness to realization: 'only after getting sick does one understand.' 健康の価値が解る ('the value of health becomes clear') uses the intransitive 解る. A proverbial observation about appreciating what you have.