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Infectious Disease Many people think of bacteria, viruses, and other microorganisms as our enemy. The germ theory of disease, first proposed in the 19th century, maintained that illness springs from the actions of infecting microorganisms. Such a view draws battle lines between “us” (the afflicted hosts) and “them” (the invading microbes). Germ theory was crucial in identifying many diseases caused by microbes and in finding ways to prevent them through such measures as immunization, sanitation, and improved living conditions. By the mid-1960s, many experts concluded that infectious disease was all but conquered, and researchers could shift their focus to chronic medical conditions such as heart disease and cancer. But this optimism was shaken in the mid-1970s and early 1980s with the appearance of Legionnaire’s disease, toxic shock syndrome, and HIV/AIDS. It was dealt further blows with the development of antibiotic-resistant bacteria and the appearance of SARS, West Nile virus, and Lyme disease. Scientists began to reexamine the relationship between hosts and microbes. Today, many scientists recognize the need for a more ecological view of the microbial world around us: Microbes and their hosts (including humans) ultimately depend on each other for survival. And although the microorganisms that cause disease often receive more attention, most microorganisms do not cause illness. In fact, many of them protect us, helping our bodies function properly and competing with harmful organisms in an eternal contest for habitable space in and on our bodies. But there is a reason why the few microbes that cause illness draw so much interest. The World Health Organization (WHO) reports that new infectious diseases are continuing to emerge and old ones are appearing in new locations around the globe. About a quarter of deaths worldwide—many of them children—are caused by infectious organisms. What’s behind this trend? How can invisible organisms cause such harm? And to what extent has human behavior amplified the problem? This booklet provides an overview of infectious disease, drawing on reports and studies from the Institute of Medicine. It describes the biology, history, and future trends of these infections. And it explains what we need to do—as individuals and as a society—to address the challenge of infectious disease.