Many serious diseases appear at first to be 'the flu'.
Even your doctor may find it hard to be sure you have just 'the flu'. It is better to get checked out carefully that to miss a serious disease that needs specific treatment. Flu sufferers usually report sudden and more dramatic onset of symptoms that often begin with a fever and tend to be less localized in the nose and throat. If sore throat is your main symptom, it is important to determine if it is a strep throat instead of a cold based on your symptoms and the findings of an examination. The doctor may want to test it for strep.