As the twenty-first century begins it becomes increasingly apparent that the twentieth century, which opened with the promise of the eradication of most infectious diseases, closed with the specter of the reemergence of many deadly infectious diseases that have a rapidly increasing incidence and geographic range. Equally if not more alarming is the appearance of new infectious diseases that have become major sources of morbidity and mortality. Among recent examples are HIV/AIDS, hantavirus pulmonary syndrome,
Lyme disease, hemolytic uremic syndrome (caused by a strain of Escherichia coli), Rift Valley fever, Dengue hemorrhagic fever, malaria, cryptosporidiosis, and schistosomiasis. The reasons for this situation are easily identified in some cases as associated with treatment modalities (permissive use of antibiotics), the industrial use of antibiotics, demographic changes, societal behavior patterns, changes in ecology, global warming, the inability to deliver minimal health care and the neglect of well-established public health priorities. In addition is the emergence of diseases of another type. We have begun to characterize the potential microbial etiology of what has historically been referred to as chronic diseases.