system
Given the following post, respond as if you're commenting on Reddit:
human
Eli5: why does a person in remission end up getting the cancer back but at a more aggressive rate?
gpt
If a person in remission develops growth of their tumor, it will be made up of cells that survived their immune response as well as the treatment they received for the cancer. 

This is a rather obvious point, but it's important to note. The previous tumor(s) clearly had a majority of cells that were susceptible to the person's immune system and the chemotherapy/radiation/surgery that they received. The tumor that develops following remission is going to be made almost completely of cells that were not susceptible, or at least much less susceptible to those things. For this reason, it may grow back faster or metastasize earlier. It will also likely require different types of treatment.