system
Given the following post, respond as if you're commenting on Reddit:
human
eli5: How does radiation both cause and cure cancer? I know it can be targeted, but I still don't understand how it's not causing cancer at it's target? How does chemo work? And how does chemo and radiation work together?

Sorry if the flair is wrong.  Biology or chemistry?
gpt
Radiation damages the DNA in cells. At smaller doses this causes mutations, which can lead to cancer. At high doses it flat out kills cells, especially cells that are in the middle of reproductions. This is why radiation poisoning causes hair loss and nausea, it killed the fast-growing cells in your hair follicles and stomach.

When used for cancer treatment, you're using targeted high doses to kill cells that are already cancerous and dividing rapidly. It's a way of killing off cancer cells that can't be removed via surgery. Nearby cells can theoretically mutate and spawn their own cancers but that's a relatively low risk compared to the risk of having an active tumour right now.

Chemo also targets cells that are in the midst of dividing/replicating...this tends to be cancer, so it preferentially targets cancer and other fast-growing cells. Hence why chemo has similar side effects to radiation. 

Together, they're two different mechanisms to kill fast-multiplying cells that can be more effective than either method alone.