One characteristic that the majority of cancers have in common is rapid uncontrolled cell division (mitosis). It is this feature that most chemotherapy agents target - they work by inhibiting mitosis. For example, azathioprine 'pretends' to be purine, which is required to make DNA. The cancer cell dutifully takes it up, only to discover that DNA synthesis consequently becomes impossible. And since DNA synthesis is required prior to any cell division, azathioprine effectively stops cancer cells dividing.
Unfortunately, most chemotherapeutic agents can't target cancer cells very specifically; that is, to some degree they affect ALL dividing cells. It's just that rapidly dividing cells are proportionally more affected. This group includes most cancer cells, but of course it isn't limited to them. Normal (healthy) cells that happen to divide rapidly become innocent victims in the whole process as well. This explains many of the side-effects:
- Hair loss is due to 'friendly-fire' on the rapidly-dividing cells at the base of the hair follicle.
- The epithelial cells of the intestine also suffer (they normally divide so quickly that the entire gut lining replaces itself every three days), causing nausea, vomiting and diarrhoea.
- Bone marrow suppression is common for the same reason, resulting in potentially low counts of red blood cells ('anaemia', causing fatigue, palpitations, etc.), white blood cells ('leukopenia', causing a predisposition to infections) and platelets ('thrombocytopenia', causing easy bleeding).
Incidentally, chemotherapy's dependency on rapid mitosis is one explanation why it is not effective for all types of cancers. Those cancers that happen to divide slowly (e.g. some lymphomas) obviously don't respond well, and other means of treatment must be employed instead - radiotherapy and/or surgery.
I really do think that some day future generations will laugh at the lack of specificity of our chemotherapeutic agents. The key to this field's future would be to identify elements specific to cancer cells, and to target these. In this way, the normal body cells would be left alone. (Antibiotics do this, and that is why, despite reaping such devastation on bacteria, they cause only comparatively mild side-effects in us.)
In this regard, some of the recent chemotherapy agents show promise to do just that. But not enough of them, and only for specific cancers.