New concepts of pathogens, and pathogenesis.

Recent changes in thinking about the host-parasite relationship are reviewed. New agents have been identified as respiratory and urinary tract pathogens. Agents previously considered benign, eg., Staphylococcus epidermidis, have become increasingly important causes of disease. Finally, studies on patients with the acquired immune deficiency syndrome have yielded important new information about both host defenses and pathogens. Continued investigation should provide important data which expand the understanding of what causes an infection and the factors responsible for disease.