Cancer was a class of diseases among humans and other species. It was characterised by uncontrolled division of cells and the ability of these cells to invade other tissues. This unregulated growth was caused by damage to DNA, resulting in mutations to genes that controlled cell division. If untreated, cancers could kill.
The First Doctor met the elderly Christian leader Hebron, who was dying from cancer. This revelation humbled him, since until then he had been immersed in self-pity over the loss of the TARDIS and separation from his companions. (PROSE: Byzantium!)
In 1976, Carolyn McConnell had dreams of finding a cure for cancer when she started out as a pre-med student in San Francisco. Some of her passion was diverted to an interest in vampires after the Eighth Doctor and Sam Jones rescued her from one of them. (PROSE: Vampire Science)
In 2011, during the Miracle Day phenomenon, the switch from mortal to immortal had rendered cancer cells mortal, allowing people to smoke freely, although the government kept this information quiet. (TV: End of the Road, WC: Web of Lies)
In the early 21st century, cancer was more common due to high pollution levels. Maria Chavez died from cancer brought on by years of exposure to toxic cleaning fluids. Stephanie had lung transplant surgery done to treat her lung cancer. (PROSE: Cat's Cradle: Warhead)