Rashes are one of the hardest symptoms to read, because very different problems can look similar. The two big buckets are allergic reactions and infections. Here is how to tell them apart — and the one test that can flag an emergency. What an allergic rash usually looks like Allergic rashes are driven by the immune system reacting to a trigger — a food, medicine, plant, metal, fragrance, or insect. Itch is the headline symptom — often intense. Comes on fast, sometimes within minutes to a couple of days of contact. Common looks: raised welts (hives) that may move around,…