Being the daughter of a nuclear physicist, I wanted to do a little explaining about the difference between "irradiated" and "radioactive".

Most people have only the vaguest idea of what radiation really is. Myths abound, like "if you get too many x-rays you'll glow in the dark". There is a difference between being irradiated and becoming radioactive. Radiation is nothing but waves (especially x-rays and microwaves) or particles (as in alpha, beta or gamma radiation). Being irradiated, i.e. exposed to radiation of either kind, will not make you or anything else radioactive. Ingesting or absorbing radioactive MATERIAL will. Thus, a hot dog that has been microwaved will not and cannot become radioactive; a hot dog that has been marinaded in plutonium will. Radiation can and does cause damage, but only when directly applied to whatever it is damaging. It cannot be transferred through an irradiated object.

Since someone asked a question about whether particle irradiation can cause things to become radioactive, I guess I'll have to explain that too. Actually, the particles themselves are not radioactive. The reason they do damage is that because of the radioactive decay of some radioactive compound they are kicked out of the nucleus at extremely high speed. They bash their way through the atoms (including those in the DNA) of your body, kind of like a bullet. The particles themselves are just photons, neutrons or electrons - which are not in and of themselves radioactive. The reason why radioactive decay can cause a chain reaction in a bomb or reactor is because the material surrounding it is unstable as well, but our bodies are composed of stable compounds which do not disintegrate because of a little extra energy.