“Distribution,” Kate said.
“Precisely,” Janus said. “The world was very disconnected. Visiting all the cultures and spreading a disease would have been impossible. A volcano that blanketed the world with ash would be perfect. On the surface, the plague might look the same. The volcano brings a winter, in some places drought then excessive rainfall. Vegetation growth plummets then rebounds. In places like northern Africa, rodent populations would fare quite well. A breeding explosion occurs. The larger population seeks new territory as their existing ecosystem can’t support their larger numbers. Some of these rats carry plague and they push into areas of human population. While the rats are immune to plague—they are reservoir hosts—the fleas on their backs are not. So fleas die of plague, and the mechanism of their demise causes them to spread the disease. Fleas infected with plague literally starve. The plague bacteria multiply in their gut, blocking the ability to ingest nutrients. They go mad, jumping off the rodents onto any host they can find, spreading the disease to humans. Of course rodents, and the fleas hitching a ride on their backs, have spread plague for thousands of years. The genius, if you will permit the term here, of this outbreak was a genetic modification to the plague bacteria—which I believe was carried by your volcano. The ash that rained down changed the bacteria residing in the rats—it didn’t unleash a pandemic on humans. A human pandemic would have simply burned out and been over with. Dr. Grey’s notation—‘Second Toba? New Delivery System?’—I believe refers to his own uncertainty on the subject. Based on our research, the work Dr. Chang and I have done, we can confirm that it was a new delivery system, an extremely ingenious one. By modifying an existing bacterial line in rats—plague—whoever did this ensured there would be multiple waves of outbreak, a sustained genetic transformation.”
David nodded. “Because the plague didn’t burn out.”
“Correct,” Janus said. “It lay dormant, in the reservoir hosts—rats in this case—waiting for the correct moment in time.”
“That matches the historical record,” David said. “The first wave of outbreak was around 535, but others followed, some even more violent. We can’t imagine the toll. The bouts of plague lasted for two hundred years. Up to half of all Europeans died. Then after about 750, the outbreaks stop until around 1257—which is the next part of Martin’s note. In 1257 another volcano erupted—again from Indonesia. These are recent discoveries, but we are pretty sure that the Samalas volcano, on Lombok Island in Indonesia, erupted with an incredible force. The impact was greater than that of the Tambora event in 1815, which caused what’s known as the Year Without a Summer. From the tree-ring samples, we see the same thing in 1257: a volcanic winter that lasted for over a year. The plague rats return and plague again returns to Europe. By this time, nearly 700 years later, the historical records are more clear. This outbreak is almost exactly like the last but it gets more press—and mention in the historical record. They call it ‘the Black Death’ in Europe. But it was the same plague—”
“Bubonic plague,” Kate said.
“Exactly,” David confirmed. “The same plague, separated by almost a millennium, returning to wreak the same havoc—”
“Stop,” Kate held up her hand. “The Black Death began in Europe around 1348—almost a hundred years after this volcano—”
“True,” David said, holding up his hands. “Look, here’s the history: in 1257 a massive volcano, strangely similar in location and effect to the one in the sixth century, caused a volcanic winter and widespread famine in Europe. I can only assume the plague returned, but there was a difference this time—some sort of immunity—”
“CCR5 Delta 32,” Kate said, lost in thought.
“What?”
“Martin mentioned it to me. It’s present in up to sixteen percent of Europeans. It’s a mutation that makes them immune to HIV, smallpox, and other viruses. Possibly the bacteria that causes plague.”
“Interesting,” David said. “One of the great mysteries of history has been the origins of the Black Death. We’re pretty sure the outbreak in the sixth century, the Plague of Justinian, moved up through Africa into the eastern Mediterranean. But the Black Death was different. Same scenario—volcano, same plague—but this time, we believe the Black Death originated in central Asia. The peace provided by the Pax Mongolica enabled the Mongol armies based in central Asia to carry the disease east along the Silk Road. During the Mongol siege of Caffa in the Crimea, the invading Mongols actually catapulted infected bodies over the city walls.”