Shadows in the lungs are imaging terms. Shadows in the lungs can be caused by a variety of reasons, except for congenital developmental abnormalities of the lungs, and usually show relative shadows on the lungs whenever there is disease in the lungs. For example, the most common cause is a lung infection, followed by a lung tumour. If you have a lung shadow, there is no need to panic, just go to hospital for a prompt diagnosis and treatment of the cause.