In some respects, the condition of Eiezer "hovering between life and death" after the camp had been liberated is unique because most of Eliezer's life in the various camps had been spent hovering "between life and death." Eliezer lived with the reality and shadow of death cast over him at all times, and was something that could not be escaped. To be "hovering between life and death," was not something new that befell him once he was sick, but rather was a state of being in the world in order to endure his time in the camps. I think that it is interesting that he would be "hovering between" both polarities in specific once the camp was liberated as it was a state that he lived with throughout his experience.
To have survived all that he has survived - the death camps, the malnourishment, the horrible conditions, the death march, only to have his life threatened after he is liberated is certainly ironic. The idea of hovering between life and death, as quoted, is also an ironic thought in that Elie has been there for years, under the Nazi regime in the concentration camps, and now finds himself near death from a relatively simple medical condition.
Add to this the idea that he went without food so often, and for so long - that malnourishment was one of his biggest threats in captivity - and yet in the end, food itself, consumed after liberation, nearly took his life. There is a lot of irony in Elie Wiesel's story.
The irony is that during the time in the concentration camp Elie had been given the worst foods that were served in filthy bowls and eaten in deplorable conditions. He had survived the horrible degrading and physically abusive treatment of the Nazi party and he had survived the gas chambers. Yet, now he has freedom at his door and the food that is served to him is healthier and better made under better conditions, and he gets deathly ill from it.
In reality what had happened to him also happened to many survivors. Their tolerance for food was so depreciated that many became ill and many died from food poisoning. Their systems could not tolerate the food after having been deprived for so long. It is ironic to survive the greatest cruelest ordeal and then to suffer near death from something that is meant to give life and nourish one's body.