The world of public health has undergone dramatic changes since the emergence of AIDS in the early 1980s. The appearance and global spread in recent years of wave after wave of new and renewed infectious diseases and their entwinement with each other and with the social conditions and biopsychological consequences of disparity, discrimination, and structural violence has produced a new significant threat to public health internationally. The term syndemic has been introduced recently by medical anthropologists to label the synergistic interaction of two or more coexistent diseases and resultant excess burden of disease. This article provides the fullest examination of this new concept to date, including a review of relevant new literature and recent research finds concerning coinfection and synergistic interaction of diseases and social conditions at the biological and population levels.