There are two types of radiation: ionizing radiation and non-ionizing radiation.  Non-ionizing radiation are things like light, radio waves, and microwaves.  These types of radiation do not normally cause tissue damage.  Ionizing radiation are things like gamma rays, x-rays, and particle bombardment and are used for medical testing and treatment, manufacturing, and sterilization.  Ionizing radiation is produced by radioactive decay, nuclear fission and nuclear fusion, extremely hot objects (thermal radiation), and accelerated charges.  Exposure to the ultraviolet radiation of the sun can damage tissue and cause skin cancer forming mutations in tissue.  Exposure to ionization radiation can cause radiation poisoning, or radiation sickness. 

When ionizing radiation collides with an atom it can dislodge some of the electrons and turn the atom into a charged particle, or ion.  When this happens to an atom in the human body it can alter the molecule that the atom is in.  This alteration of molecular make-up interferes with cell division and can cause problems for cells that are normally rapidly dividing, like those cells in the gastrointestinal tract.   Ionizing radiation will also reduce the  amount of blood cells and platelets, and results in an increased tendency to bleed and depresses the body's defense against infections.  It can also cause cellular lesions, certain types of cancers and diseases, and other malfunctions in cells.  Prolonged exposure to radiation can also cause premature aging and mutations.  Extremely high doses of ionizing radiation to the body can cause massive tissue damage to the brain and death will ensue within 48 hours.

Ironically, radiation therapy is also used to treat cancer.  Radiation’s ability to disrupt cell division is useful for removing cancerous tumors.  Studies have also shown that low levels of ionized radiation can lower one’s risk of certain types of cancer, in a process called hormesis.