The progression and effects of cancer.

(a) In a healthy individual, an initial gene mutation converts a normal cell into a tumour cell. This tumour cell divides to produce a benign tumour. Additional genetic changes in the tumour cells may occur, leading to a malignant tumour. At a later stage in malignancy, the tumour cells will invade surrounding tissues, and some malignant cells may metastasize by travelling through the bloodstream to other parts of the body. (b) On the left is a photo of a human lung that was obtained from a healthy nonsmoker. The lung shown on the right has been ravaged by lung cancer. This lung was taken from a person who was a heavy smoker.