system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How exactly does radioactivity cause damage to the human body, and how do the symptoms result?

I want to know what really goes on there in the cells and radiation. All I know is that gamma rays have an extremely low fequency and can go through your body...
gpt
Gamma rays are very *high* frequency, which is what gives them the oomph to mess things up. Gamma and X-rays are "ionising" (electromagnetic) radiation, which is radiation that has the energy to hurt your body by breaking chemical bonds and forming highly reactive free radicals. Obviously, this is not good for your body. So radiation hurts you by causing physical damage to your body at very small scales (destroying cells, damaging DNA). Radiotherapy for cancer is based on this principle: they irradiate the tumour to destroy its cells.

There are three kinds of radioactivity that hurt you. Alpha particles, which are big and heavy (relatively-speaking) and *really* smash up your body's cells, but can't penetrate very far. Beta particles (basically, an electron), which are much lighter, but can penetrate further, and electromagnetic radiation (X-rays and gamma rays). Lower frequency electromagnetic rays, e.g. microwaves are non-ionising. That's why mobile phones, which use microwaves, *don't* cause brain cancer.

An aggravating factor in the toxicity of radioactive materials is how the body reacts to the substance. Iodine is stored by the thyroid gland, especially in children (which is why everyone should take iodine pills after a nuclear incident—the thyroid will be full and the radioactive iodine will be excreted by the body), while plutonium is deposited in the bones. Your body can get rid of uranium, however.