。
The real issue is how to prevent the disease.
Literal
True [possessive-の] problem [topic-は] how that disease [object-を] prevent [question-か] [copula-である].
どうやって~か ('how to do X') is an embedded question functioning as the predicate via である. The entire clause acts as the answer to 'what is the real problem?' — it's a content question embedded in a declarative sentence.