What Is Cancer?

April 1, 2026

This page was reviewed under our medical and editorial policy by Vijay Trisal, M.D., surgical oncologist and chief clinical officer for the City of Hope^® system.

Cancer is a disease that develops when cells in the body start to behave abnormally. In most cases, cancer is the result of genetic changes that cause the body’s cells to divide and grow out of control, forming masses called tumors. There are more than 100 types of cancer and the disease may develop in almost any part of the human body.

Cancer cells inside a tumor may also spread to other parts of the body, either by causing the cells in nearby tissue or organs to become cancerous or by traveling to other areas of the body through the bloodstream or lymphatic system. The process of cancer cells spreading throughout the body is called metastasis.

Cancer Cells Versus Normal Cells

Normal, healthy cells are controlled by a process called cell division. This process ensures that cells in the body grow normally and that any unhealthy or damaged cells die off to be replaced with new cells. If the cell division process is disrupted, then cells may start to grow uncontrollably and form a tumor. Tumors may be noncancerous (benign) or cancerous (malignant).

One way doctors diagnose cancer is by looking at a sample of cells taken from a patient’s body. This is because cancer cells and healthy cells look different when viewed under a microscope, as explained below.

Noncancerous cells: When cells are healthy, they appear fairly regular in shape and size, their nuclei (or “control centers” in the middle of the cell) are roughly the same size and they are in the location they are supposed to be within the body’s tissue.

Cancerous cells: When cells become cancerous, they may appear irregular in size or shape, have enlarged or misshapen nuclei and have started to spread or move to other layers of tissue in the body.