One study that concerned me at the time said that people who repeatedly bit their cheeks could form cancer in that spot. In my pre-braces days, that was happening to me. A prevalent theory of how cancer occurs says that whenever cells are injured or die, the process that repairs or replaces them can make mistakes. A very small percentage of those mistakes can cause a cancerous cell to occur. Let's backtrack a bit and introduce some definitions.
Cancer is a family of diseases that is characterized by cells that replicate too rapidly.
It is called a family of diseases because there are many different types of cancer and most of them have different characteristics. The cancerous cells often form a structure that is called a tumor , though some cancers don't, such as leukemia, which produces too many white blood cells that are then released into the bloodstream. The unrestrained growth of cancerous cells is caused by a defect in their DNA . DNA is made up of genes , which contain the blueprint of every part of our bodies, including plans for how cells grow. Every time normal cells reproduce, they replicate their DNA and this becomes the basis of the new cells.