Radiation sickness is damage to your body caused by a very large dose of radiation often received over a short period of time (acute). The amount of radiation absorbed by the body — the absorbed dose — determines how sick you'll be.
Although radiation sickness is serious and often fatal, it's rare. Since the atomic bombings of Hiroshima and Nagasaki, Japan, during World War II, most cases of radiation sickness have happened after nuclear industrial accidents, such as the 1986 nuclear reactor accident at a power station in Chernobyl, Ukraine