system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What is deadly about cancer ?
gpt
Three big things:

* Cancer grows really, really fast and uses up a lot of nutrients in doing so
* Cancer abandons all the established rules of cell growth and may grow to a size that physically interferes with the function of other organs (half your lung being cancerous isn't good for your ability to breathe)
* Even if a cancer isn't in the worst imaginable location, the potential for metastasis hovers over your head constantly - that it might spread to a more vulnerable region of the body like the brain.