Variation in surface antigens has been well recognized as a mechanism by which pathogenic organisms can avoid elimination and remain as potential pathogens in immunocompetent individuals. A variety of viral and parasitic organisms elude the immune system by varying their surface antigenic structures. Other persistent human pathogens, for example group A streptococci, are associated with cyclic variation in the severity of infections without any major change in their surface antigenic structures. Recent analysis of group A streptococcal proteins, in particular surface M and M-like proteins, has documented the existence of an array of multifunctional surface proteins which have the ability to bind to a variety of normal human plasma proteins, extracellular matrix components and human cells. The ability to change the functional activities of these surface molecules by genetic recombination among members of a closely related M protein supergene family has now been reported. In this paper, the potential importance of generating functional heterogeneity in surface binding proteins of group A streptococcus is discussed. The role of these proteins in enabling an organism to sense its environment and express the appropriate virulence factors is proposed as an explanation for the periodic changes in the frequency and severity of invasive group A streptococcal infections that can occur in the absence of a toxic-shock-like syndrome.