system
Given the following post, respond as if you're commenting on Reddit:
human
ELI 5: when someone has cancer, what exactly kills them?
gpt
Basically, cancer is out-of-control cell growth. Normally cells are pretty good at regulating their growth, and they stop growing when they're supposed to (e.g., when your liver is the size it's supposed to be, it stops creating new liver cells, except as needed to replace the old ones).

Cancer cells have the parts of their DNA that control cell growth damaged, so they just keep growing and growing.

Normally they start out confined to one place, so you just get a big lump of cancer cells, called a tumor. Then some cells start breaking off and spreading throughout the body, and they seed new tumors all over. This is called metastasis, and this is where things get bad. The tumors all over your body just keep growing. Some of then grow inside your organs, or near your organs, and start crushing them. This interferes with the normal function of your organs, and eventually kills you.