I have noticed that in the many lovely notes I have received from friends recently that almost to the person everyone refers to my battle against cancer. I have used a similar metaphor myself. But I wonder why we have adopted fighting words when it comes to disease, particularly one like cancer.
Cancer is your body punking you. Cells that should die do not and grow into places they should not. To stop that process, you have to cut out the offending cells, radiate them out or kill them and other healthy cells with toxic chemicals. The battle, if there is one, is against yourself and your out of control uncooperative adolescent cells.
The metaphor could be merely a function of a society obsessed with violence. Or it could reflect that there is something awful out there (in my case, the chemo as much as the cancer) that needs to be overcome. One could use a mountain climbing metaphor or a marathon metaphor or something else to show you are overcoming an obstacle. The chemo kills the cancer and undermines the host for the cancer. Yet the common wisdom has the battle as being mine to win or lose.