Today, more than 2 billion of the planet's 6 billion people are infected with the latent form of tuberculosis. In the United States alone, 10 to 15 million are infected with it. Of this multitude, at least 10 percent of them will go on to develop the active form of tuberculosis sometime during their lives. The period between latency (with no signs of disease at all except a positive TB skin test) and active illness can range from weeks to years after the microbe Mycobacterium tuberculosis settles into the human body. Much has to do with the health of the particular host, his nutritional status, living conditions, and the coexistence of other diseases. But when considering the many influences that enable the TB bacillus to transform good health to ill, it is wise to recall the French chemist and microbiologist Louis Pasteur's warning to respect the "infinitely great power of the infinitely small."4

Every year public health officials across the globe diagnose more than 8 million active cases of TB. These are the people who are most infectious to others. The sicker someone is with active TB, the greater the number of microbes in his or her body; and with every cough, shout, or breath, that individual becomes a more significant risk to the public health. The average person with active TB will infect twenty other people before he or she dies or is adequately treated. What surprises many is that during every twelve-month period, about 3 million people die of tuberculosis, making it the leading infectious cause of death in the world today. In fact, more human beings will die of TB in our era than at any other point in recorded history.

The disease, often referred to as the white plague in deference to the infamous bubonic, or black plague, typically strikes adults during their most productive years of life, ages eighteen to fifty-five. Fortunately, since the late 1940s medicine has been blessed with a wide range of potent antibiotics that can treat TB and, in most cases, cure it. At the same time, these medical miracles have given rise to premature declarations of victory over tuberculosis. Especially over the past decade, public health specialists have grown concerned about the rise of multidrug-resistant strains of tuberculosis around the world. In layman's terms, this means that not only do we have deadly germs on our hands, but, in at least one out of ten cases (and far more in Asia, South America, and the former Soviet Union), the mycobacteria are resistant to the very menu of powerful drugs we have developed to enter and kill them.

Only recently have we begun to recognize the folly of extensive funding cuts in tuberculosis surveillance and treatment programs, and the unintended consequences of not investing in the basic health care needs of the most impoverished citizens of the world. Moreover, with the rise of HIV/AIDS over the past two decades, there is now a significant population of immunocompromised individuals who are highly susceptible to TB and can potentially spread the disease like an uncontrollable wildfire. Major social upheavals during these years have also resulted in mass migration movements around the globe.

Each of these factors has created the conditions the tubercle bacillus needs to thrive once again, inspiring the World Health Organization to classify tuberculosis as "a global health emergency."5 My fellow subway passengers that afternoon may have been worried about the off chance of meeting an anthrax spore. A few may have been contemplating the risk of a terrorist delivery of smallpox virus or even the highly unlikely importation of Ebola virus. I was, and am, far more concerned about contracting TB.

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Excerpted from When Germs Travel by Howard Markel Copyright © 2004 by Howard Markel. Excerpted by permission of Vintage, a division of Random House, Inc. All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.

Tags: Disorders and Diseases, Infectious Diseases

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About the Author

Howard Markel is the George E. Wantz Professor of the History of Medicine and Professor of Pediatrics and Communicable Diseases at the University of Michigan, where he directs the Center for the History of Medicine. He is the author of the award-winning Quarantine! and numerous articles for scholarly publication, as well as for The New York Times, Harper's, The Atlantic, The Washington Post, and National Public Radio. More