Cancer

Cancer was a class of diseases among humans and other species, characterised by uncontrolled division of cells and the ability of these cells to invade other tissues. This unregulated growth is caused by damage to DNA, resulting in mutations to genes that control cell division. If untreated, cancers may eventually cause death.

Real world
Several Doctor Who contributors have died from cancer over the years:
 * 1981 - Victor Carin
 * 1982 - John Bay
 * 1993 - Jacqueline Hill
 * 1994 - Peter Cushing
 * 2001 - Tom Watson, George Harrison, Kevin Manser
 * 2002 - Carmen Silvera
 * 2006 - Gary Downie
 * 2007 - Dave Martin
 * 2011 - Elisabeth Sladen