Emerging and re-emerging infectious diseases continue to represent a major worldwide public health problem. Naturally occurring and bioterrorist introduced agents can result in disease and death. Societal, technological and environmental factors have a significant effect on infectious diseases facilitating the emergence of new diseases and the re-emergence of old ones. Recent occurrences that underscore the public health concerns include viral infections, such as HIV, SARS, NIPAH virus, dengue, SLE, West Nile virus and bacterial infections, such as Helicobacter pylori, E. coli 0157: H7, and antibiotic resistant organisms including tubercle bacilli, staphylococci, streptococci, and gram negative rods. The role of infectious agents as causative factors of malignancies will be reviewed. We will discuss the general factors that have influenced the occurrence of these diseases and the programs promoted by the Centers for Disease Control and Prevention, the World Health Organization, and other professional organizations, to combat these infectious diseases. The major emerging and re-emerging infections will be discussed emphasizing their identification, epidemiology, recommended control and prevention measures, and their current unresolved problems. We will also discuss the threat of bioterrorism in reintroducing an infectious agent.