Cancer is a disease which is caused by a loss of the ability of one or more cells to control mitosis, usually resulting in the growth of tumours - disorganised collections of abnormal cells which can reach considerable size.
The tumours can block ducts and passageways, for example in the lungs, and/or prevent the normal functioning of organs of the body, whilst parts can break off, move - usually carried in the blood - and lodge elsewhere (metastasise) causing the rapid spread of the disease.
These days, many cancers can be successfully treated by a combination of removal of tumours with other therapies which target abnormal cells around the body.
The onset of cancer results from changes in the DNA which affect the genes controlling cell division, usually in a single cell at first.
A very modern disease, not all causes have been identified, but include cigarette smoke and exposure to excessive radiation such as sunlight or X-rays.