system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: how does cancer return back more aggresively after treatment?
gpt
You’ve killed the weaker cancer cells and the ones that remained were the strong ones. It’s like antibiotic resistant bacteria, except instead of bacteria being selected by the antibiotic, it’s a population of cancer cells being selected by the treatment.  

Also the next time it comes back your immune system is probably weaker as a result of the cancer treatment itself.