In 1914, a German microbiologist named Walther Kruse gained the first solid clue about the origin of colds by having a snuffly assistant blow his nose. Kruse mixed the assistant’s mucus into a salt solution, poured it through a filter, and then put a few drops of the filtered fluid into the noses of 12 colleagues. Four of them came down with colds. Later, Kruse did the same thing to 36 students, and 15 of them got sick. While he ran this experiment Kruse also kept track of 35 people who didn’t get the drops. Only one of them came down with a cold on their own. Kruse’s experiments made it clear
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