system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How exactly does cancer kill you?

Obviously it will kill you if it overruns a vital organ, but is it just as simple as obstructing normal bodily functions?
gpt
Cancerous cells will do a few things that are dangerous. First, they divert resources to themselves. They take up sugar and oxygen from your bloodstream and give nothing in return, which weakens your body and leaves it vulnerable to infection. Sometimes tumors even cause blood vessels to divert to them in a process callef angiogenesis. One of the warning signs for cancers are sores that don't heal, because your body is so weak it can't do simple repairs anymore. Second, cancerous cells will invade and attack surrounding tissues, damaging them and impairing function. Thirdly, tumors gunk up your system and physically impair normal function. A tumor in the brain can put pressure on the brain, restricting blood flow and causing brain damage. Leukemia can be so bad the blood becomes watery and pus-like because healthy cells are getting crowded out by cancerous cells. Finally, some cancers are malignant, meaning the start in one place and then colonize other parts of the body. For example, breast cancer might be bad, but if it stayed in the breast it probably wouldn't kill you. However, if cancerous cells migrate out of the breast to vital organs, the risks shoot up.