Common wisdom has it that when you're sick, you should "starve" a fever. Your appetite may be nonexistent, but when you have a fever, you're burning more calories than you normally would. While you're fighting off an infection, your body needs nutrients and energy to bolster your immune system. [2] When the friendly kind of flora flourishes in your gut, your immune system hums along healthily. FruitsWhen you have a fever, fruits are your friend.