system
Given the following post, respond as if you're commenting on Reddit:
human
How does cancer actually kill you?
gpt
There's some variation between different types of cancers, but we can think of their effects at the tissue level, organ level, and systemic level.  At the tissue level, cancers can be locally invasive, with upregulation of various factors that allow it to penetrate through tissue planes and break down normal connective tissue barriers.  In doing so, a cancer might eat through a part of the GI tract leading to a perforation and sepsis, or invade into a major blood vessel and cause severe bleeding.  Cancers can also cause a mass effect just by the size of the tumor impinging on neighboring structures.  This may cause occlusions of vital passageways (commonly seen in the GI/hepatobiliary tracts), or increase pressure on sensitive areas such as the brain.  At the organ level, cancers that grow in vital organs can severely disrupt their function.  A cancer of the liver may severely impair the synthetic and metabolic function of the liver, leading to coagulopathies and encephalopathy.  A tumor in the lung can physically and mechanically prevent it from functioning as a gas exchange interface.  A cancer in the bone marrow can reduce the body's ability to make red blood cells, white blood cells, and platelets.  Note that both tissue level and organ level dysfunction may be at the site of the primary tumor or at a metastatic location.  At the systemic level, many cancers can cause disease through their paraneoplastic effects.  Many cancers have profound effects on the hormonal and endocrine milieu of the body and can lead to severe derangements in electrolyte levels, neurotransmitter signalling, etc.  Also remember that a malignancy represents a state of high stress for the body, which can tax its ability to deal with other comorbidities.  In addition, infections are a common problem for these patients because the immune system is often impacted, either directly or secondarily through chemo.  Along this vein, the treatments we have for cancer are rarely benign, and may include invasive surgeries, high doses of radiation, and what are in effect therapeutic poisons.