Cancer is a type of disease where cells grow out of control, divide and invade other tissues. In a person without cancer, cell division is under control. In most tissues, healthy cells divide in a controlled way and copy themselves to create new healthy cells. With cancer, this normal process of cell division goes out of control. Cells change their nature because mutations have occurred in their genes. All the daughter cells of cancer cells are also cancerous.
If the abnormal cells do not invade, but just divide and swell up their original tissue, this is not called "cancer". It is called a tumour. Tumours are usually not a threat to life because they can be cut out. However, some tumours occur in places where they cannot be cut out, and they can be fatal. Some brain tumours are of this type.
The symptoms of cancer are caused by the cancerous cells invading other tissues. This is called metastasis. Metastasis is a process in which cancer cells move through the bloodstream or lymphatic system. When this happens, a person's cancer can be spread throughout his or her body. Eventually those other tissues cannot work as well, and the whole body begins to get worse, and may die.
Cancer can affect anybody at any age. Most types of cancer are more likely to affect people as they get older. This is because as a person's DNA gets older, their DNA may become damaged, or damage that happened in the past may get worse. One type of cancer that is more common in young men, rather than older people, is testicular cancer (cancer of the testicles).