What can cause hives?

So Many Causes: There are a number of potential causes of hives (urticaria) which include: viral infections, parasitic infections, IgE-mediated allergic reactions like bee stings, medication allergy, latex allergy, food allergy, transfusion reactions, and contact reactions. Some meds/products can induce hives like narcotics, radiocontrast media, muscle relaxants, stinging nettle, physical stimuli, and foods.

Many things: Would see an allergist who can best determine the cause if one is obvious. In most cases a cause is never found.