Exposure to animals, especially cats. Eating eggs, fruits, nuts and shellfish. Other foods sometimes cause hives in infants, but not in adults. Other allergies or a family history of allergies. If a medication or acute viral infection is responsible, hives usually disappear within hours or days.
Some cases become chronic and last for months or years.
Most eventually go into spontaneous remission even if the cause is not identified. Your own observation of symptoms. Medical history and exam by a doctor. Diagnostic tests may include laboratory blood studies, urinalysis, erythrocyte sedimentation rate and chest x-ray to rule out inflammatory infection. Allergy skin tests and injections.