“Regular radiation” such as microwave, infrared and visible light typically doesn’t have the energy needed to break chemical bonds, so we may sit out in the sun and get bombarded with a thousand watts and feel no ill effects. Once we reach ultraviolet though, this radiation now has enough energy to break those chemical bonds –including the ones in our bodies. This means that high energy radiation such as that emitted from an x-ray tube can damage DNA, and in high enough doses may even cause radiation sickness.
Acute radiation sickness occurs when your body has absorbed a large amount of ionizing radiation, usually on the order of several sieverts. What makes radiation lethal is the effect it has on DNA. When a high energy particle, be it a photon or some other particle collides with DNA it breaks bonds and rearranges the bases. Normally your cells can repair this damage, but if a cell fails at that task it often commits suicide before it divides. For long living cells such as muscle this isn’t too much of a problem, since the other cells have time to replace the dead ones. For short-lived cells though, this apoptosis becomes a major issue as cells are dying too fast to be replaced.
Such short lived cells include the mucus-making cells that line the intestinal wall. When exposed to enough radiation, these mucus cells start to die off en masse, and so are not replaced. No mucus cells means there will be no mucus, and no mucus means there is no protection from stomach acid. The intestine stops absorbing food particles, acid burns the tissue, and eventually you die of sepsis. If somehow you survive this ordeal, you will now need a bone marrow transplant since the short-lived bone marrow cells have died off. Radiation sickness symptoms include nausea, stomach pain and a lack of energy, and a detailed chart of symptoms can be found here
And that’s why we shield ourselves from ionizing radiation! Keep in mind that it takes a very large amount of radiation to cause radiation sickness, not something a fiestaware plate or even a radium painted clock could ever produce. However, a Coolidge tube is certainly capable of generating very intense radiation!