6 Diseases Back from the Dead

Plague, TB, and measles seem like illnesses of the history books or infections beaten back by modern science. But once-vanquished diseases are now reemerging all over the world. Here's why. By Stephanie Warren

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Plague

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.

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