Did rats really spread the Black Death?

Partly, but researchers still argue over how much. The textbook chain is rat, flea, human. The bacterium Yersinia pestis lives in rodents and in their fleas, and humans are infected mainly through the bite of the oriental rat flea. The story goes back to Alexandre Yersin, who found the bacterium in 1894 during a plague epidemic in Hong Kong and suggested the rat was the main carrier, and to Paul-Louis Simond, who showed in 1898 that fleas pass it on.

The flea part is gruesome and clever. The bacteria multiply in a flea's gut until they block it. The starving flea bites over and over, trying to clear the blockage by regurgitating, and in doing so it flushes thousands of bacteria into the bite. When the rats that carried the disease die, their fleas look for new hosts, and that includes people.

But some researchers think the chain was too slow for the Black Death. They point to evidence that once it came ashore, the disease spread mostly from person to person as pneumonic plague, which would explain why it moved inland faster than rat fleas alone would predict. Others proposed in 2018 that human fleas and body lice did much of the work. Academic debate continues, but no single alternative explanation has won wide acceptance.

The cause itself is not in doubt: Y. pestis DNA has been found in teeth from Black Death mass graves.