context: Radiation sickness happens when a large dose of high-energy radiation goes through your body and reaches your internal organs. It takes far more than what you might get from any medical treatment to cause it. Doctors named the illness, which is technically known as acute radiation syndrome, after the atomic bombings that ended World War II. It's not clear how many of the 150,000 to 250,000 people killed in those attacks died from radiation sickness. But estimates at the time put the number in the hundreds or thousands. Since then, about 50 people have died from radiation sickness. That includes 28 workers and firefighters who were killed at the 1986 nuclear accident at Chernobyl in what is now Ukraine. More than 100 others at Chernobyl were diagnosed with acute radiation syndrome but survived. Most of the other people who have died from it were scientists or technicians in U. S. or Soviet nuclear plants during the Cold War. But in 1999, three workers got radiation sickness after an accident involving nuclear fuel in Japan; two of them died. No cases of radiation sickness were reported after the Fukushima Daiichi nuclear accident in 2011. The amount of radiation your body gets is measured in an international unit called a sievert ( Sv). Symptoms of radiation sickness show up when you're exposed to levels of more than 500 millisieverts (m Sv), or half a sievert. More than 4 to 5 Sv is likely to be fatal. The workers who got radiation sickness at Chernobyl received doses that measured 700 m Sv to 13 Sv. Natural radiation is everywhere -- in the air, the water, and materials like brick or granite. You typically get only about 3 m Sv -- three one-thousandths of a sievert -- of radiation from these natural sources in a year. Man-made sources of radiation from things like X-rays add about another 3 m Sv. A CT (computerized tomography) scan, which involves several X-rays taken from different angles, delivers about 10 m Sv. People who work in the nuclear industry aren't allowed to be exposed to more than 50 m Sv a year. The most common early symptoms of radiation sickness are the same as for many other illnesses -- nausea, vomiting, and diarrhea. They can start within minutes of exposure, but they may come and go for several days. If you have these symptoms after a radiation emergency, you should seek medical help as soon as it's safe to do so. You might also have skin damage, like a bad sunburn, or get blisters or sores. Radiation may also damage the cells that make hair, causing your hair to fall out. In some cases, hair loss might be permanent. The symptoms can go away entirely for anywhere from a few hours to weeks. But if they come back, they're often worse. Radiation damages your stomach and intestines, blood vessels, and bone marrow, which makes blood cells. Damage to bone marrow lowers the number of disease-fighting white blood cells in your body. As a result, most people who die from radiation sickness are killed by infections or internal bleeding. Your doctor will try to help you fight off infections. He might give you blood transfusions to replace lost blood cells. Or he may give you medications to try to help your bone marrow recover. Or he may try a transplant. He also will give you fluids and treat other injuries like burns. Recovery from radiation sickness can take up to 2 years. But you'll still be at risk of other health problems after recovery. For example, your odds of getting cancer are higher.