"The first stage had been all over before the doctors even knew they were dealing with a new sickness; it was the direct reaction to the bombardment of the body, at the moment when the bomb went off, by neutrons, beta particles, and gamma rays"

John Hersey's quote assesses the instant and terrible effect of atomic warfare, specifically in the context of the atomic battles throughout World War II. Hersey is explaining the first phase of radiation sickness, a direct result of exposure to the extreme levels of radiation produced when an atomic bomb detonates. The phrase "before the physicians even understood they were dealing with a new sickness" stresses the extraordinary nature of this medical phenomenon. At this point in history, radiation sickness was reasonably unidentified, and physician were unprepared for the special challenges it presented.

The "direct response to the barrage of the body" describes the instant effects that high dosages of radiation have on the body. This can include damage to cells and tissues, manifesting as nausea, vomiting, and fatigue in the short term, and more extreme impacts such as burns and organ failure over time. Hersey is highlighting the fast onset of these signs, as they take place at the very minute the bomb releases its energy.

The reference of "neutrons, beta particles, and gamma rays" specifies the kinds of ionizing radiation that are given off throughout a nuclear explosion. These particles and rays permeate the body, ionizing particles and causing cellular damage that leads to the symptoms of radiation sickness. This clinical detail highlights the complexity and invisible nature of the risk, posturing a plain contrast to the standard injuries from weapons known to the medical neighborhood at the time.

Hersey's chilling description serves not only to inform but also to stimulate a deep compassion in the reader for those who experienced these scaries firsthand. It underscores the profound and instant transformation of reality triggered by nuclear warfare, marking a critical point where humankind was required to reckon with the destructive capabilities of its own technological improvements. In essence, Hersey catches both the scientific unfamiliarity and the psychological gravity of this new type of human suffering.