system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do people with severe radiation sickness get that period where they suddenly get completely fine before getting worse again?
gpt
The cell life cycle exists in stages. Prior to cell division the cell does a self-check of the DNA to verify it's all where it should be to ensure healthy cell division. If the DNA is damaged and cant be repaired then apoptosis occurs, programmed cell death.

When exposed to high levels of radiation some cells are severely damaged while others are only somewhat damaged. The severely damaged ones die off relatively quickly. Then healing occurs which triggers "healthy" cells to divide to replace the dead cells. Those "healthy" cells go through their checks to ensure successful division. This is when the DNA damage is discovered and the cells kill themselves, resulting in the delayed, more severe cell die off.