Attacked by the school bully, the backpack fell from Abner’s shoulder.
A dangling modifier is a modifier that has nothing to modify. Abner was attacked by the school bully, not the backpack as the sentence suggests because of this error. In other words, we have here a dangling modifier or misplaced modifier (Attacked by the school bully).
It could mean that WWI created many more open opportunities for a disease to be spreaded because it was a corrupt time when many people had to fight in the war together live together etc. to survive which made many people more prone to catching diseases that people may have had.