次の文は間違っていないが、不自然な文である。なぜ不自然かを説明してください。後は、文が自然になりうるコンテキストを考えてくれませんか?
先生がインフルエンザをうつしてくれたので、学校を休まなければならなかった。
This sentence is grammatically correct but semantically weird. This sentence has the subsidiary verb くれる, which indicates the teacher did something good for you. In this case, you were negatively affected by the teacher, so くれる is not an option. so you should use the passive form rather than くれる. This type of passive form is particularly called "suffering passive". Here is the corrected version:
先生にインフルエンザをうつされたので、学校を休まなければならなかった。
(Note for more advanced learners: In fact, the sentence in question is not absolutely incorrect because くれる is sometimes used when someone did something bad for you. See Using くれる for doing something bad for details. This type of くれる sounds arrogant and derogatory. Beginners should not use くれる in this situation even when the teacher is not present.)