The word severe is the most appropriate collocation for illnesses like fever or pain to describe intensity. It specifically refers to something very intense or serious in nature.

১. Strong attack is generally used for physical force or military actions.
২. Serious attack is grammatically correct but not the standard medical idiom.
৩. Bad attack is a very casual expression and lacks formal precision.

অতিরিক্ত তথ্য: Severe fever or severe pain are common medical descriptions used in English.