When abnormal cells grow out-of-control they form tumors that are either benign (not cancerous) or malignant (cancerous). All cancers are formed this way including lung cancer, which is formed in either or both lungs. Normal cells follow a rhythmic process that produces new cells while the old cells die out. Cells become abnormal when this process is interrupted. As the body produces new cells, the old ones do not die out fast enough causing damaged DNA cells in the form of tumors, except for leukemia which is formed in the blood.
Hope is being able to see that there is light despite all of the darkness. ~Desmond Tutu