Answer: Because they had never developed immunities to the disease
<em>Basically the text is explaining that the way Americans level of skill made them unequaled to the rest of the world (mind, that is not a fact because there was a large share of well-working people in the world in this time) . Also it explains that American brain and ability to construct (why is this entire article just bragging about Americans being so much better when that's not 100% true?) was one of the most economic boosting things and they were very productive for that timeframe.</em>
<em>That's my best understanding of the article. (Scholastic really likes to brag about Americans, huh?)</em>
<em>-Northstar</em>
