。
At first I thought it was an illness.
Literal
At-first [topic-は] illness [copula-だ] [quotative-と] thought.
A simple reflective statement. 初めは 'at first' topicalizes the initial time period, implicitly contrasting with a later state when the speaker realized otherwise. 病気だと思った 'thought it was an illness' uses the quotative と to embed the judgment as the content of thought. A compact narrative setup hinting that the truth turned out to be different.