Infectious diseases remain a serious medical burden in both developing and industrialized countries. The emergence of new diseases such as HIV/AIDS and severe acute respiratory syndrome (SARS), the resurgence of known diseases such as West Nile disease and multidrugresistant tuberculosis, and the threat of deliberately developed man-made infections are cause for concern. When vaccines or effective treatments are not available, we rely on the immune system to clear the host of infectious agents and disease. A better understanding of the tactics used by pathogens and of the immune system's defensive armoury is needed to pave the way for improved strategies of prevention and therapy. The four articles in this Insight provide a snapshot of the issue, addressing in turn the nature and spread of emerging and re-emerging infectious diseases, the assault strategies used by major pathogens, the immediate innate immune response, and the antigen-specific acquired (or adaptive) immune response, which is the host's second line of defence.
