> Sure we know what causes cancer, uncontrolled growth of a mutated cell!

That is an effect, not a cause. Your claim is like saying that auto accidents are caused by cars getting too close together. That's only a symptom of an underlying cause.

> I see where you're coming from, but the differences between pneumonia and cancer aren't as great as you think.

We know what causes pneumonia. Because of this, we can suggest behaviors that will prevent it before the fact, we can identify it unambiguously, and we can cure it after the fact. This isn't true about cancer -- we don't know what causes it, therefore we cannot proactively prevent it, we can't identify it very efficiently (there are always false negatives and positives) and we cannot cure it. There's no comparison.

> They can test for the presence of the pathogen, but they can't ever say you are "cured".

Of course they can. In infectious disease, they can test for the presence of the causative pathogen -- no pathogen, no disease. In cancer, to do this, we would first need to identify the cause. But we don't know the cause, we only know the effect.

> In cancer you can test for the presence of the mutated cells (minimum residual disease).

This is like saying we can test for the presence of a car crash by measuring bent bumpers and inflated airbags. That measures effects, not causes.

> Often what they'll do is count the mutated cells from a biopsy, and with this current treatment, the number of remaining cells is less than the lower limit of detection.

Counting abnormal cells only reveals how little we know. We detect cancer by detecting abnormal cell growth. We measure progress in symptomatic treatment by counting abnormal cells. We declare a remission by making that count approach zero using agents that kill abnormal cells. There are any number of cases where the count of cancerous cells was below the limit of detection, but still caused a recurrence.

In 1979, smallpox was declared to have been globally eradicated. How? By identifying the responsible pathogen, then systematically destroying it everywhere it appeared. This was only possible because we know what caused smallpox, and we could treat the cause, not the symptoms. Because of what we knew, we were able to call smallpox permanently cured.

We cannot do this with cancer, because we don't know enough about how it works. All we can do is tell people to avoid risky behaviors, behaviors that, for often-unclear reasons, increase one's chance to contract this disease. Then, once the disease's symptoms appear, we have crude methods to deal with it, like trying to kill the cell masses that represent the disease's primary symptom.

Based on a comparison with other diseases, cancer treatment is unbelievably primitive -- it would be like treating a finger infection by cutting off a person's hand. And guess what? Before germ theory and before antibiotics, that was the treatment -- cut off the infected limb before the infection spread through the body.

The reason we don't tell people they're cured of cancer is because of science. In science, there are no fairy tales -- everything depends on evidence. And we don't have the evidence science requires to declare cancer cured.