system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: If our body creates memory cells for immunity after we have an infection or virus, how come cancer survivors are more likely to contract cancer again and die from it?
gpt
Cancer isn't bacteria or virus, it's you body's own cells malfunctioning. Normally your immune system can notice when cells are reproducing uncontrollably, but not always. They are your cells, not foreign bodies.

Even when tumors get destroyed, there are often still rogue cancer cells floating around in your blood that get missed. They're just too tiny to catch all of them, usually, so there is a chance they'll re-implant somewhere down the line and begin reproducing again, growing a new tumor.