。
Curing illness is the job of doctors.
Literal
Illness [object-を] curing [topic-のは] doctor [possessive-の] job [copula-である].
病気を治すのは uses the nominalizer の + は to make 'curing illness' the topic. 医者の仕事である asserts it as 'the work of doctors' with the formal copula. The ~のは...である structure is common in expository writing.