There are several things that distinguish a healthy cell from a cancerous cell. Cancerous cells reproduce much more rapidly and haphazardly, they don't heed messages from nearby cells that attempt to curb this reproduction, they never die, and they don't stay where they're supposed to. The healthy cells, on the other hand, are the body's team players -- they remain in place, reproduce when there's room to do so, and kill themselves off when they present a danger to the greater good. They have on and off switches that regulate their behavior.
The abnormal behavior of the rogue cells can lead to cancer. These cells take on a life of their own, grouping together to cause tumors and traveling throughout the body, spreading the cancer.
Every cell in our body has the potential to become cancerous. We're born with the genetic coding endowed to us by our parents, and sometimes those genes have flaws in them which predispose a cell to cancer. Then, throughout our lives, our genes are subjected to environmental factors and simple bad luck, either of which can cause a change in the cell that may eventually lead to cancer. Smoking and sunlight are examples of environmental factors that can affect our genes, but these are factors that we consider controllable. Sometimes, though, changes to the genetic code are due to unavoidable chance. Our cells divide and reproduce many times over the course of a lifetime, and each time this occurs, the genetic code must be copied exactly. Given the complexity of the information, as well as how many times division occurs, a few changes are inevitable.
These genetic changes, be they hereditary, environmental or haphazard, are known as mutations, and when enough random mutations are allowed to accumulate within a cell, it turns cancerous and exhibits the behavior that we described at the beginning of this article. Mutations indicate that some of a cell's information has been damaged or lost, and without that data, the cells stop acting normally. These cells no longer have those on and off switches that keep them in check in relation to other cells. They may start reproducing uncontrollably because they lack the proteins that would stop such behavior, or they may lose the ability to repair slight changes, which most cells are able to do. With each additional reproduction and division, the mutations continue and the genetic material becomes even more convoluted.
How these cancerous cells behave is dependent on where in the body they formed and the specific mutations they've acquired, but the process from healthy cell to cancerous cell is similar for all types of cancer.