The most common are coronary heart disease and tobacco-induced cancers. Another group of conditions are traffic-related injury causing premature death or permanent disability; and intentional injury -- suicide, homicide, domestic violence -- that may be a manifestation of underlying malaise in society, akin to what Durkheim called anomie. These changes in patterns of disease in the second half of the 20th century have provided a raison d'etre and a good living for legions of epidemiologists; my specialty has made many important contributions to understanding of causes and ways to control these "new" public health problems. But now we are facing newer public health problems: infectious diseases have come roaring back. Both old microbial enemies and new ones threaten us. Emerging and re-emerging infectious pathogens are and probably always will be one of the greatest threats to human health.