Population structure of pathogens: The role of immune selection

Sunetra Gupta and Roy Anderson discuss how the balance between host immune responses against conserved and variable antigens shapes the population str ucture of pathogens. At one extreme, immune selection against polymorphic d eterminants can cause pathogen populations to self-organize spontaneously i nto discrete antigenic types that may either be maintained over long period s ou undergo cyclical or chaotic fluctuations. At the other extreme, divers ity may he drastically reduced by competition induced by a strong immune re sponse against a conserved determinant. Where does each pathogen lie along this continuum? How would this knowledge influence our attempts to control an infectious disease?