Cancer is a group of diseases characterized by uncontrolled cell growth. Cancer begins when a single cell mutates, resulting in a breakdown of the normal regulatory controls that keep cell division in check. These mutations can be inherited, caused by errors in DNA replication, or result from exposure to harmful chemicals. A cancerous tumor can spread to other parts of the body and, if left untreated, be fatal.
Cancer, often known as "the Big C". Cancer is not really one disease, but it's a group of diseases, and what they share in common is the feature of uncontrolled cellular growth. Now, cancer in the body normally begins many, many, many years before your doctor actually gives you a diagnosis. It begins when a single cell collects enough genetic mutations that it results in a breakdown of the normal processes that keep cell division in check. So in its simplest terms, cells go wild, and they just start dividing over and over and over in a very uncontrolled way, eventually resulting in a tumor. The mutations may be inherited. That is, you could've gotten them from your mother or your father's DNA. They can be caused by errors in DNA replication, because the machinery in the cell that's responsible for duplication doesn't always work perfectly. Or it could be because of exposure to harmful chemicals; something that happened that you were exposed to when you were a child that you or your family have absolutely no recognition or recollection of. A cancerous tumor can spread to the other parts of the body, and that's called metastasis. And generally, if left untreated, cancer can often be fatal.