I've just copied and pasted this from my junior hons dissertation (getting a bit dusty now). I hope it's not too boring; I remember finding this area really interesting (and shocking) at the time.
'Another area in which war metaphors are employed is in the fight against disease. In her book Illness as Metaphor and AIDS and Its Metaphors, Susan Sontag noted that the military metaphor first came to be used in this context in the 1880s, when bacteria were first discovered and described as ?invading? and ?infiltrating? (2002: 67). Sontag?s book focuses for the most part on the metaphors of cancer, and the military metaphor appears to play a large part in the characterisation of this particular disease: ?every physician and every attentive patient is familiar with, if perhaps inured to, this military terminology? (Sontag 2002: 65). The cancer itself is seen as an invading force, in phrases such as ?malignant tumours invade even when they grow very slowly,? which has its origins in a medical textbook (Sontag 2002: 65). Treatment, therefore, must be equally aggressive; Sontag notes that radiotherapy uses metaphors of aerial warfare, e.g. ?bombardment?, and that the aim is to ?kill? cancer cells (2002: 66).
The metaphor of war in the fight against illness can also have negative consequences, according to Sontag:
??the move from the demonisation of the illness to the attribution of fault to the patient is an inevitable one, no matter if patients are thought of as victims. Victims suggest innocence. And innocence, by the inexorable logic that governs all relational terms, suggests guilt.? (2002: 97)
As such, Sontag?s research led her to believe that the war metaphors used in the treatment of cancer led the patient to be stigmatised along with the ?enemy? illness that inhabits them.'
I think it stems from fear. People are understandably terrified of cancer; I suppose that, subconsciously, it's comforting to think that it won't happen to you, because you'd fight it. It gives the illusion of control, I suppose.