Urticaria is usually triggered by allergens, such as drugs, foods, insect bites, inhaled substances, in response to sunlight, cold, heat, stress, exercise, or a viral infection
. Other common allergens are pollen, mold, dust, pet dander, or cosmetics. Since your sister experienced an itch, just tell her to go over these possibilities in your mind. Something may strike her. In many cases, hives clear up themselves and don’t require diagnosis or treatment.