Cancer is not caused by bacterial infections or a virus, like so many other diseases. Cancer happens when our own cells start to grow and divide abnormally (although some infections can create conditions in which our cells are encouraged to turn cancerous). By the time a cancer is large enough for modern medical science to detect it, your body has already thrown many defensive weapons against it. In fact, finding and destroying cancerous cells is part of the body's everyday routine. Cancer as we know it happens when the body's defensive weapons are losing the battle against that abnormal growth. That's when we have pull out the big guns: surgery, radiation, and/or powerfully destructive chemicals.
That our immune systems go to work constantly to fight the body's own cells when they become destructive is astonishing in itself. Some of our cells have tiny testing labs looking for odd proteins, along with a library of those proteins, plus we have an army of killer cells detecting abnormalities in different ways. So not only does our immune system look for cells that shouldn't be there (like bacteria and viruses), they also inspect all our cells to make sure they stay in line. Kurzgesagt explains how that works at the cellular level, and even further down in our DNA.