Cancer is actually a group of many related diseases that all have to do with cells. Cells are the very small units that make up all living things, including the human body. There are billions of cells in each person's body.
Cancer happens when cells that are not normal grow and spread very fast. Normal body cells grow and divide and know to stop growing. Over time, they also die. Unlike these normal cells, cancer cells just continue to grow and divide out of control and don't die when they're supposed to.

Cancer cells usually group or clump together to form tumors (say: too-mers). A growing tumor becomes a lump of cancer cells that can destroy the normal cells around the tumor and damage the body's healthy tissues. This can make someone very sick.
Sometimes cancer cells break away from the original tumor and travel to other areas of the body, where they keep growing and can go on to form new tumors. This is how cancer spreads. The spread of a tumor to a new place in the body is called metastasis.

Causes of Cancer
You probably know a kid who had chickenpox — maybe even you. But you probably don't know any kids who've had cancer. If you packed a large football stadium with kids, probably only one child in that stadium would have cancer.
Doctors aren't sure why some people get cancer and others don't. They do know that cancer is not contagious. You can't catch it from someone else who has it — cancer isn't caused by germs, like colds or the flu are. So don't be afraid of other kids — or anyone else — with cancer. You can talk to, play with, and hug someone with cancer.