Cancer kills in many ways. Depending on the specific type, all or some of the following may apply:
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Cancer, at its primary site can cause lethal problems eg. lung cancer blocking a bronchus (breathing tube) leading to pneumonia, eg. cancer of the bile ducts blocking off the liver
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Cancer can spread locally and kill, eg. a lung tumor that eats it way into a blood vessel or into the heart lining
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Cancer can spread far beyond its primary site and do damage there, eg. breast cancer spreading to the liver and causing liver failure, or eg. breast cancer spreading diffusely through the lungs, or eg. lung cancer spreading to the brain
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Cancer weakens the body, sometimes in obvious ways such as malnutrition, and sometimes in subtler ways such as inhibiting the immune system. In either case, infections result and are much more severe than in “normal” individuals.
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Cancer makes the blood clot more readily. Blood clots kill. (The association between cancer and clots is quite common but not generally appreciated, IMO, by lay people)
(N.B. Both #5 and #6 are made worse by the immobility of advanced cancer patients)
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Cancer replaces the bone marrow causing fewer white blood cells and/or platelets to be produced. Infection and/or bleeding results.
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Cancer can leads to bizarre, unpredictable, and sometimes mysterious effects in the body, eg. lung cancer causing “Lou Gehrig’s Disease”, eg. kidney cancer causing high blood calcium levels, eg. myeloma (a type of bone marrow cancer) causing breakdown of the nerves in the body (neuropathy), eg. lymph cancer leading to kidney failure
I’m sure there are even more mechanisms.