Hand, foot, and mouth disease is a mild viral illness that can occur at any age, occurring most often in children under age 10. The disease is characterized by vesicles (small blisters which contain clear fluid) that occur inside the mouth, on the gums and on the side of the tongue. On rare occasions persons with the virus that causes hand, foot, and mouth disease may develop viral meningitis. Infants who develop oral vesicles may stop nursing and become dehydrated. Most cases of hand, foot, and mouth disease occur in summer and fall.