system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: how/why does radiation cause illness?

An earlier post asked what was radiation which was answered by explaining particles and waves and degradation etc. So to follow from that, what is it about these loose atoms that cause cancer and other illnesses (or for that matter, how does it cure stuff too?)
gpt
Ok - so based on the earlier post, you know that radiation is little "bits" of matter that are moving at high speed, or light that's a very high energy.  Both are carrying a lot of energy, and that energy is the problem.

Living things are made up of little "bubbles" called cells, and each cell is alive.  At the heart of the cell is something called DNA, and it's special.  It's special because DNA is like a really long book that tells the cell how to do everything it needs to do.

Cells eventually "divide", or split into two cells.  This is how we grow bigger, and how we heal when we're hurt.  Some diseases, like cancer, are what happen when cells get out of control and start dividing like crazy.

When radiation strikes DNA, it's carrying so much energy that it can damage the DNA, so that the instructions in it are now wrong.  Usually it means the cell will die.  Sometimes, the change makes the cell forget how to stop dividing, so it keeps going - this is cancer.

Now, cells are so small that, aside from cancer, damage to a single cell isn't going to do much.  You need to kill a lot of cells, all at once, for you to notice.  A lot of radiation will damage a lot of cells, and that's where "radiation sickness" comes from.  This affects different body parts in different ways, and I'll explain why.

When cells become damaged by radiation, they don't usually die right away.  They usually die when they try to divide.  Not only that, but cells have a way to fix the damage caused by radiation.  So, if a lot of time passes between the radiation and when the cell divides, there's a pretty good chance that the damage was fixed, and the cell will not die.

Some parts of the body need cells to keep dividing all the time.  The cells that make up your skin and make hair are always dividing, but brain cells don't divide very much once you're all grown up.  So, if you get a lot of radiation, your hair falls out, but your brain is ok.  

The faster the cells are dividing, the more sensitive they are to radiation.  But remember that cancer divides really really fast - out of control?  That means that if you expose cancer to radiation, it's not going to have time to fix the damage, and it will die.  That's how we use radiation to treat cancer.  Surrounding healthy cells might die too, but less of them will die than the cancer cells, because they're dividing more slowly.  Because only some of the healthy cells die, you don't notice, and you're just glad you don't have cancer any more.