Blackbody radiation
A single wave or photon of infrared light is very low energy, cannot go through the human body, and can only warm it. Planck's good thinking was to realize that to get a single wave at the x-ray frequency, it was necessary to have a big enough package of energy (or "quantum") to make such a strong wave. So if a blackbody took in a single wave at the x-ray frequency, then it could give off an x-ray at some later time. But if the blackbody only took in infra-red light it would not matter how much of it was absorbed.