Neat post. Conceptually, single-celled organisms can't get "cancer" because, in a way, they are cancer. However, they no doubt can suffer mutations or other genetic changes (like from viruses) that make them survive and reproduce more or less well, all things considered for their current environment. Cancer has to do with a cell deciding not to play nicely with the rest in a body, and to strike out on its own, so to speak. Cancer in general is a bit like a crazy individual or small group in a society trying to take over the whole thing (current US plutocrats?); generally it works out badly for everyone as core services start to fail and the cancer cells are no longer supported by the rest of the body. Cancer is like spammers, who for a quick buck in the short term, are busy destroying email and the rest of the internet that could otherwise bring everyone abundance. Cancer is about "selfishness" where the individual ignores its part to play in the whole and where the whole supports the individual. But since evolution involves variation and selection, the underlying mechanism of cancer via mutation or viral infection also in a sense underlies evolution. So yes, it will always be with us.
I've heard most people in the USA age 40+ years old have cancerous cells in small amounts, but the immune system is continually killing them off to keep them from spreading.
Good nutrition helps with that, like Dr. Joel Fuhrman talks about
"Though most people would prefer to take a pill and continue their eating habits, this will not provide the desired protection. Unrefined plant foods, with their plentiful anti-cancer compounds, must be eaten in abundance to flood the body's tissues with protective substances. Vegetables and fruits protect against all types of cancers if consumed in large enough quantities. Hundreds of scientific studies document this. The most prevalent cancers in our societies are plant-food-deficiency diseases. The benefits of lifestyle changes are proportional to the changes made. As we add more vegetable servings, we increase our phytochemical intake and leave less room in our diets for harmful foods, enhancing cancer protection even further. Let's review some of these research findings and then review what a powerful, anti-cancer diet will look like. ... A typical anti-cancer diet should contain at least 4 fresh fruits daily, at least one large raw green salad, as well as a two other cooked (steamed) vegetables, such as broccoli, carrots and peas, squash or other colorful vegetables. A huge pot of soup laden with vegetables, herbs and beans can be made once a week and conveniently taken for lunch. Raw nuts and seeds are another important, but often overlooked, group of foods with documented health benefits contributing to longevity. ..."
One thing Fuhrman misses in his discussion is that these compounds are not "Anti-cancer" as much as the human body has adapted via evolution to use these compounds to prevent or fight cancer.
He is right that cancer is best prevented rather than treated. As I've heard, it said, you can either get your chemotherapy every day from fruits and vegetables, or you can end up getting it all at once in the oncologist's office (not that most current chemotherapy is probably worth it anyway).
Fasting may also sometimes help prevent cancer as well as can a ketogenic (fat burning) diet that deprives cancer cells of sugar.
But your point stands that this is all combinatorial (statistical, entropical?) about when something gets out of hand. Even when we have Elysium-like medical beds that get rid of cancer instantly, some computer virus or malicious person may make them work incorrectly. Or, as in the movie, selfish elites can keep the healing beds to themselves.