system
Given the following post, respond as if you're commenting on Reddit:
human
Why is radiation contagious?

I understand radioactive decay and how that can damage cells by polarizing the DNA.

What I don't understand is how a non-radioactive material can become radioactive to a dangerous point.  I would think that the material would get hit by the decaying particles, get damaged and that's it.  Instead, the material can also become radioactive.

For example, Marie Curie's notebooks are reportedly still too radioactive to be safely handled.  How does paper become radioactive just by being exposed to radiation?
gpt
There are two ways in which objects can be 'activated':

The first is that radiation from the original radioactive material hits the nuclei of the object, and through the reverse process of radioactive decay create an unstable nucleus. This however very rarely happens, unless you put something in a very intense radiation field (near the core of a nuclear reactor for instance).

What probably happened with Curie's notebooks is that radioactive material got transferred to her hands, and from her hands to her notebook. Where they still radiate. This is why radiation hygiene is very important when dealing with radioactive materials. For instance, the same radioactive particles were probably transferred to food that she than ingested. Inside her body is where the radiation could do most damage.