The correct answer is "Parents refused to care for their dying children".
The situation to which the narrator is referring to as "even worse, and almost incredible" during the plage, is that parents were abandoning their suffering children. This is what he found to be the most poignant and striking image that conveyed the extent of the horrors brought by the plague. Mothers and fathers refused to care for their dying children or even visit them in innumerable cases.
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