“Perhaps the most difficult thing to come to terms with is the scale of death.” This particular statements gives the main idea of the paragraph. The other sentence can not be interpreted when broken, while as, after reading the first sentence, anyone can easily grab the motif and central idea of destruction by the subject (epidemic). Moreover, readers can easily suggest that something very dangerous is being discussed in the excerpt which is a hazard to multiple lives.
Interestingly, after reading the complete paragraph, we all understand the theme is what is presented in the very starting lines of the extract: "Perhaps......of death."