The Black Death struck Europe in 1348. Just five years later, a third of the population—50 million people—was dead. And though plague seems like an illness of another time—that it belongs to a day when a victim was treated with a healthy bloodletting and a garlic-onion-butter poultice—it's still with us today. Small outbreaks of plague can swiftly strike a few hundred people before they can be stopped. Researchers are trying to figure out why the disease was so deadly in the Middle Ages, and how it's causing harm once again.
To answer the first part of that question, scientists from McMaster University in Hamilton, Ontario, recently sequenced the genome of the bacterium responsible for the Black Death. They drilled into the soft tooth tissue of four victims recovered from a London Black Death gravesite called East Smithfield, and got enough DNA fragments to map the killer's full genome for a study in Nature.
What they found was very strange. Since outbreaks of the modern plague, while scary, are nowhere near as devastating as the Black Death, researchers thought the modern plague would show significant changes in its DNA when compared to the ancient one. But instead, today's plague is barely different from its centuries-old ancestor. The factors that probably made medieval plague outbreaks so severe include the unsanitary living conditions of those days, and perhaps a population that was genetically predisposed to being more sensitive to the disease. But paper co-author Kirsten Bos says, all these years later, scientists don't have the full picture. "We can't explain why we saw that cataclysmic mortality."
Plague is primarily transmitted to humans by fleas that live on rodents. Modern plague outbreaks have occurred in California, New Mexico, Arizona, and Colorado, where wild rodents carry the disease. Other hot spots for modern plague outbreaks include South American countries such as Ecuador and Peru, and parts of Africa and Asia.
Most of these outbreaks take the form of bubonic plague—a type that causes grossly swollen lymph nodes. Because it's easily treatable with antibiotics, this kind is the least deadly. But the other forms of plague—septecemic, which infects the blood, and pnemonic, which strikes the lungs—are much worse. Pnemonic plague spreads directly from person to person, and attacks so quickly that patients can die before antibiotics take effect. Without early treatment, pnemonic plague's death rate is nearly 100 percent. "If you have an outbreak of pnemonic plague, that is going to be bad news for that community," says Juan Olano, M.D., an infectious disease expert at the University of Texas.
So far authorities have been able to contain plague outbreaks. But some diseases don't stay confined.
In 2000, 304 men and women in peak physical shape landed in Malaysia for the adventure of a lifetime. They trekked through jungles, mountains, and rivers to compete for the top spot in the Echo Challenge, a multiday adventure race called the toughest endurance event in the world. When they boarded the plane for the return trip, 29 of them unknowingly carried a dangerous bacteria called Leptospira, and fell mysteriously ill when they got home.
Leptospira lives in many mammal species. It enters water sources through infected animals' urine, and it can pass to humans if they swallow infected water or if it enters through a skin wound—something the battle-scarred athletes had in excess. The infected athletes were hospitalized for fever, nausea, and vomiting before doctors figured out what was wrong with them; luckily, none died. Outbreaks, however, continue to occur worldwide—in the U.S., the disease usually shows up in people who have recently traveled.
Diseases move at the speed of their hosts. As we become increasingly globalized, we give the bugs a big advantage: They can zip from place to place as fast as a jetliner. "You can travel from London to New York and carry with you an infection that you don't even know you have," says David Heymann, chairman of the U.K.'s Health Protection Agency.
Human hosts aren't the only thing infectious bacteria can hitch a ride on. "Mosquitoes are carrying infections around the world," Heymann says, so much so that they've inspired the term "Airport Malaria" (the disease-laden insects hitch a ride on a plane). When they deboard in their new location, they can infect someone even if that person has never left the country. Airport Malaria has become a problem in nearly every country, Heymann says, and leptospirosis and malaria aren't the only diseases that have become increasingly dangerous because of our globe-trotting lifestyle.