Radiation is a special kind of energy carried by waves or a stream of particles.
It can come from radioactive substances and special machines. There are many different levels of radiation energy. We have all been exposed to some low levels of radiation energy, such as radiation from the sun or radiation used to make a chest x-ray or mammogram. When radiation energy is used at high levels, it can treat cancer and other illnesses. The use of high-energy rays or particles to treat a disease is called radiation therapy .