I'm not a doctor, but my understanding has been that we all have cancer cells at any given point in time. Consider how many cells inside you are undergoing cell division; errors in cell division are where cancer cells come from.

The question is whether your body sees and kills the cancer cells; most of the time it does, which is why most of us can live pleasantly for most of our lives despite errors cropping up silently. What we term cancer are the cancer cells that slipped past our body's defences, growing into masses that cause no end of grief.

To put it in computer terms, it's like error correction in HDDs and SSDs and ethernet connections. Errors are inevitably going to occur while data is in transit, but they are of no concern so long as error correction and other such mechanisms can correct and recover. The errors do become a concern when they start slipping past such mechanisms, however.