Don't omit that "that"

> He tried to isolate the germ he thought was causing the disease.
> 我认为 i thought 是插入语，插入语在句中不做任何成分，所以这句话在
> was前是不是缺了一个that 呢？

I agree. The word "that" should not be omitted.

Also see this:

"There're too many people make such mistakes."

Indeed many Chinese say that because it's a direct translation from Chinese. The word "that" (or "who") after "people" should not be omitted. Or just say "Too many people make such mistakes."