All the time you are in doubt about the cause of your illness, you are fighting against an invisible enemy.

一直以来你都在怀疑自己的病因，你是在和一个无形的敌人斗争。

I'm rather in doubt about his explanation of this question.

我对他对这个问题的解释相当怀疑。

There seemed to be no doubt about it.

此事似乎毫无疑问。

There is some doubt about the best way to do it.

这件事怎么做才是最佳办法有点吃不准。

There is no doubt about her suitability for the job.

毫无疑问，她适合做这个工作。