Pathogenesis: A History of the World in Eight Plagues

by Jonathan Kennedy

Although endemic pathogens killed large numbers of Romans, they also had a surprising benefit: diarrheal disease and malaria created what amounted to a protective force field around the imperial capital. Anyone who had survived until adulthood would have acquired immunity but people who came from outside, including those who wanted to conquer the city, were at high risk of getting sick or dying if they stayed too long. Malaria had prevented attacks on the imperial capital since at least the late third century BCE, when Hannibal managed to cross the Alps with 60,000 troops, 12,000 horses and 37 war elephants. The Carthaginian army routed the Romans on several occasions. It was malaria, which killed Hannibal’s wife, son and many of his soldiers, that brought the invasion to an end and saved Rome.[27] But infectious diseases only worked in Rome’s favor for so long. The pax Romana produced conditions that were ideal for the emergence of epidemic infectious diseases. The growth of long-distance trade with sub-Saharan Africa, India and China increased the risk that Romans would come into contact with novel pathogens—and the well-connected, highly urbanized Roman Empire created a perfect breeding ground for fast-spreading new diseases. It was only a matter of time before an outbreak brought this unprecedented period of peace and prosperity to a sudden halt.