Allergic Reaction: This is the most common cause of hives, often triggered by allergic reactions to foods, medications, or environmental factors. The sudden onset of hives after exposure to a potential allergen supports this diagnosis.

Insect Bites or Stings: Reactions to insect bites or stings can cause hives, especially if the individual has an allergy to the venom.
Viral Infections: Certain viral infections, such as mononucleosis or hepatitis, can cause hives as part of their symptomatology.
Autoimmune Disorders: Conditions like lupus or rheumatoid arthritis can sometimes manifest with hives due to the body's immune response.
Food Allergies: Allergies to specific foods, such as nuts, shellfish, or dairy, can lead to hives among other symptoms.

Anaphylaxis: Although rare, anaphylaxis is a life-threatening condition that can cause hives, along with other severe symptoms like difficulty breathing and a drop in blood pressure. It requires immediate medical attention.
Mastocytosis: A condition characterized by the accumulation of mast cells in the skin, which can lead to hives and other systemic symptoms.
Urticarial Vasculitis: An inflammation of the blood vessels that can cause hives, but also has the potential for more serious complications if not addressed.

Thyroid Disorders: Both hyperthyroidism and hypothyroidism can rarely cause hives, though this is less common.
Cancer: In rare instances, certain types of cancer, such as lymphoma, can present with hives as part of a paraneoplastic syndrome.
Hereditary Angioedema: A genetic disorder that leads to recurrent episodes of severe swelling, including hives, but typically without the itching associated with allergic reactions.