Hives occur when your body releases histamine and other chemicals in response to various triggers. The causes can be diverse, ranging from allergic reactions to foods, medications, or insect stings and bites. Environmental triggers, such as pollen, animal dander, and mold, can also provoke hives. Physical triggers, including heat, cold, pressure, or sunlight, may lead to their appearance, as can emotional stress, which sometimes brings on hives. Infections, like the common cold, flu, or other viral and bacterial illnesses, are another potential cause. Additionally, chronic conditions such as autoimmune diseases, like lupus, or thyroid problems, can contribute to hives. In some cases, however, the exact cause remains unidentified, a condition known as idiopathic urticaria.