Is extreme fatigue due to cancer, or just what happens during cancer treatment?

Sometimes yes: Cancer steals your body's energy, taking nutrition away from the rest of your tissues and most importantly your brain. This can lead to fatigue. Or, it could be the treatment; chemotherapy can definitely cause fatigue.

Both: Cancers are highly active cells and they consume the resources of the persons body very fast. Cancer treatments can also make a person tired due to damage to normal cells that occurs and the repair process. This is a very simple explanation of the reason why.

Both: Fatigue may be presenting symptom of many cancers. Treatment with chemotherapy and radiation add to the fatigue.