Who is the doer when we use the verb 崩す? is a transitive verb so it should have a doer(omitted or not) In this sentence, the doer seems to be , he destroyed himself with alcohol: > Drink brought about his downfall. But in the following sentence, it doesn't work well to say that is the doer since it seems that he is not voluntarily ill: > , . He has been off school with a slight illness this week. Is the doer omitted in this sentence? If yes, is a sentence like correct to say that he is ill because of a cold? I don't find any occurrence on google search so I think it is not natural but I don't know why in this case?

> {}{}{}{}, {}{}{}

Without a doubt, the subject/doer of the verb phrase would be , but that is not to say that voluntarily chose to become ill. You might have to forget the translated English word "destroy" here because that could fool you in your attempt to determine the "doer".

is the subject/doer of both actions and in this sentence. There is no unmentioned subject/doer here.

> "He has become slightly sick and has been absent from school this week."

You ask:

> is a sentence like {} **** correct to say that he is ill because of a cold?

No, it is not correct, grammatical or natural-sounding. You can say instead:

> ****

using a .