Germs

History is full of events waiting for someone to notice the connection between one piece and another. That's one of the things that makes history so interesting — there's always something new to learn, to notice.

Here's an example, taken from an article about historian William H. McNeill in the University of Chicago Magazine:

McNeill first noticed disease lurking in the shadows of historical documents when he was researching The Rise of the West. Like accounts of the Antonine Plague, historical records made passing mention of disease. But there was little analysis of their role in shaping history. "I read the story of Cortéz and couldn't believe it," McNeill says. The conventional story of how Tenochtitlan [modern-day Mexico City] fell to Hernán Cortéz and a small band of Spaniards in 1521 seemed to contradict common sense. At one point, the Aztecs had beaten the Spaniards back but did not press their advantage. "A considerable number of the Spanish were wounded in the retreat but there was no follow-up," McNeill says. "I couldn't figure out why the nephew of Montezuma, who organized the attack, didn't surround the Spaniards and bring them up to the top of that temple and cut their hearts out the next day. It's what should have happened."

And yet, it didn't. Instead, the Spaniards conquered Mexico and converted millions of Aztecs to Christianity. "I was sort of mulling this over in my head," McNeill recalls, "and somebody casually remarked that smallpox had broken out in Mexico City the night of the noche triste"—the night of the Spaniards' retreat—"and Montezuma's nephew died of it that same night."

The plague struck only the Aztecs. The Spaniards, who had developed an immunity to smallpox during repeated childhood exposures, were spared. The implication in the eyes of everyone who lived through the Aztec epidemic, McNeill argues, was the superior power of the Spaniards' God over the Aztecs' deities. "The whole history of the New World hinged on that episode," McNeill says. Suddenly he realized "there was a whole history that had to be written.

Sometimes you just need one more piece of information for an event to make sense.