病気だったので

because [someone] was sick

→ 病気
phrase Rare #22,253
because [someone] was sick 病気だった means “was sick,” and ので means “because” or “since.” The subject is understood from context.
JA: 病気だったので、学校を休みました。
EN: Because I was sick, I stayed home from school.

彼は病気だったので彼らは静かにしていた

He was sick, so they kept quiet.

私は病気だったので行かなかった

I didn't go because I was sick.

私は病気だったので仕事に行けなかった

I was sick, so I couldn't go to work.

私は病気だったので家にいなければならなかった

I was sick, so I had to stay home.

彼は病気だったので来られなかった

He was sick, so he couldn't come.