Daniel Hauser, the 13-year-old boy who was ordered by a judge to undergo chemotherapy against his parents' wishes, has an advanced case of Hodgkin's lymphoma, a cancer that develops in the lymph system. Because lymph tissue is found throughout the body, Hodgkin's lymphoma can start almost anywhere and spread anywhere. The lymph system includes:

Lymph: The colorless fluid that carries throughout the body white blood cells, which fight infection and disease.

Lymph nodes: Small, bean-shaped structures that filter lymph and store white blood cells. They are found along the network of lymph vessels and cluster in the underarm, pelvis, neck, abdomen and groin.

Spleen: An organ that filters the blood, stores blood cells and destroys old blood cells. The spleen is on the left side of the abdomen.

Thymus: An organ in the chest behind the breastbone where white blood cells grow.

Tonsils: Two small masses of lymph tissue that make white blood cells.

Bone marrow: The soft, spongy tissue in the center of large bones. Bone marrow makes white blood cells, red blood cells and platelets.

Sources: National Cancer Institute; Up to Date