Comment by asdff

We already have these nanobots in our bodies. This is the entire rub with cancer. There is amazingly strong selective pressure to evolve ways to hide from our existing nanobots and make them think these are valid healthy cells. Artificial nanobots would be no different in this respect. As for why our nanobots aren't already perfect, it's hard to get perfect. In fact it can go the wrong way, some people's nanobots end up too aggressive and start attacking their healthy cells (autoimmune disorders).