Source: Wikipedia, Author:BruceBlaus, Credit:CC BY- SA 4.0
Apart from radiation syndrome excavated in individuals who escaped from the atomic bombing of the Hiroshima and Nagasaki cities of Japan between the 6 and 9 of August 1945, launched by the United States Army Air Force, there is barely any recent documented case of acute radiation sickness except that which was discovered in people who were involved in nuclear power plant accident at Chernobyl Ukraine in late 1986.
People who are affected by atomic bombs or individuals imperiled to a bomb are called Hibakusha (coined from the Japanese language).
Acute radiation syndrome (ARS), is observed in individuals who are exposed to a high dose of radiation within a short period of time. It occurs when the body gets accumulated with a high dose of ionizing radiation which penetrates the body for a short time. ARS is rare, initial cases were detected in individuals who survived the atomic bombing of the two cities of Japanese and the nuclear power plant accident at Chernobyl Ukraine.
Before I proceed, let us shed little light on what ionizing radiation is and how dangerous it is when absorbed by the body.
According to World Health Organization, >'' ionizing radiation is radiation with enough energy so that during an interaction with an atom, it can remove tightly bound electrons from the orbit of an atom, causing the atom to become >charged or ionized”

The massive amount of energy produced from this reaction is termed as radiation. This is to explain that the breaking away of electrons from an atom is ionization and like I said, the chemical reaction is being accomplished by a huge amount of energy. Bombs are created by utilizing the above-mentioned process. Atomic bombs consist of unstable atoms like uranium or plutonium, when the electrons in these atoms break, there is usually a vast amount of energy released to the environment. Particles like Alpha and Beta which do not have a high penetrating power will not induce any significant harm to humans but will do so in some certain conditions while X-ray and Gamma-ray will initiate irradiation to an uncovered body since they have great penetrating strength. The splitting up of the elements is called a fission reaction.
The energy so released from the above-explained reaction, when absorbed into a body in high dose can cause serious effects to any of the detected organs of the body.
A skin burn caused by exposure to high ionizing radiation
Source: wikipedia, Author: LK Wagner, PhD; Vlietstra et a, Credit:CC BY- SA 3.0

Our body is a build-up of many cells and these cells are very sensitive to ionizing radiation, so exposure of the body to any radiation source for a short period of time, depending on the dose acquired will generate fallout to the affected organ.
According to prudent researchers, a body exposed to ionizing radiation for a long time accumulates a lesser dangerous dose than that, exposed to irradiation for a short period of time. This hypothesis has not been challenged since absorption of a high dose of ionizing radiation over a short period of time evokes skin burn or acute radiation syndrome while low dose received for long periods of susceptibility stimulates a long-term disease like cancer according to WHO.
Note that naturally, we do experience a small proportion of radiation in our daily lives. Probably, from the food we consume which contains radioactive material or from the air we breathe or from cosmic rays, but our system is proficient to respond and maintain the internal environment so that we will not be harmed. Those radioactive elements consumed along with food, when deceased, the body eliminates them via urine. Again, the body of adolescents can easily be penetrated by ionizing radiation than adults and will arouse genetic mutation if occurred in the genetic cells hence can be passed on to the fetus which will induce mental challenge when developed.
Now back to the topic, ARS is prompted by a high dose of ionizing radiation when the body is exposed for a short period. The body is being victimized based on the dose acquired and the duration of exposure. Ionizing radiation dose is measured in a unit called Gray.
1-2 Gray or 100-200 rads: moderate
3-6 Gray or 300-600 rads: severe
7-8 Gray or 700-800 rads: very severe
10 Gray or 1000 rads: very dangerous and may not be treatable.
Dose below 0.1Gy will not inflict significant harm to the body but from 1Gy to 10Gy or higher gray. The susceptible body will exhibit symptoms counting on the dose absorbed and the period of time as well. This is to assert that initial signs and symptoms revealed in a body exposed to ionizing radiation is determined by how long the body was exposed and the dose accumulated,(the higher the dose, the rapid symptoms evoke).
So, when a whole or part of the body absorbed 10Gy or 1000rads or even more, there is rapid symptoms development and the exposed body may die within 3 days.

Traditional, Acute radiation sickness is divided into three syndromes.

Bone Marrow syndrome:

Bone Marrow syndrome will ensue when the bone is exposed to a high dose (between 0.7Gy or 10Gy and 70rads or 1000rads) of radiation. The blood cells are manufactured in the bone marrow and irradiation of the marrow will prompt the destruction of the bone marrow hence, there will be a low level of blood cell production. An insufficient amount of red blood cells in the system caused by radiation exposure will stimulate anemia, a state whereby the system is incapable to control bleeding. This can sometimes lead to death if care is not taken. Treatment includes medication with filgrastim.

Gastrointestinal syndrome:

This syndrome will evoke when there is a high dose of 6-30 Gray (600-3000)rad accumulated. The fallouts discerned from irradiation of the gastrointestinal are nausea, vomiting, dysplasia, abdominal pain, and diarrhea. Occasionally, symptoms may become more apparent between 6Gray or 600 rads. In this case, there is a chance of survival but death primarily occurs due to enteritis other than gastrointestinal dysfunction.

Neurovascular/ Central Nervous System(CNS) syndrome:

Dose like 30 Gray or 3000 rads or more will ostensibly incite this syndrome. Sometimes, signs will improve between 20 Grays (2000 rads). Abrupt discernible symptoms involve dizziness, headache, and fallout in the conscious. Note that, the higher the dose, the rapid the symptoms evoke.

Predominantly, symptoms of acute radiation syndrome are categorized under four serial stages.

• The prodromal stage(N-V-D stage)

The symptoms found in this stage are nausea, vomiting diarrhea, and anorexia, which will develop after divulgence for some instants and can prevail for some days

• Latent stage

Symptoms in this stage can not be discernable and the affected person doesn't show any signs of unhealthy for some consecutive days or even for some weeks.

• Manifest illness stage.

Symptoms advancement depends on the category of syndromes( bone marrow, gastrointestinal, or neurovascular syndrome) anticipated.

• Recovery or death stage.

In this case, patients who could not recover after a few months or for some years will eventually die.

Since acute radiation syndrome can be spotted with no unique disease, it is hard to diagnose and there is no cure for the sickness. Patients are relatively placed on medications depending on the dose absorbed. Blood transfusions and biotics, stem cell transplant, or removing any radioactive material discovered on the skin or accumulated in the body can boost the survival of any ARS victim.
In conclusion, long-term fallout like cancer prevails in individuals who experienced low ionizing radiation either by accident or intentionally.