A version of this story appears in the September 2020 issue of National Geographic magazine. Philadelphia detected its first case of a deadly, fast-spreading strain of influenza on September 17, 1918.

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Les Masterson is a deputy editor and insurance analyst at Forbes Advisor. He has been a journalist, reporter, editor and content creator for more than 25 years. He has covered insurance for a decade, ...

STEP 2 - Plot the coordinates (2,3). Remember the x (horizontal) is the first number in the brackets and the y (vertical) is the second number. Each axis is labelled with a scale of numbers, like a ...