A particular strain of the group A streptococci (GAS), known as the M1T1 clone, has exhibited unusual epidemiology and virulence, and has been largely associated with the pandemic of invasive GAS infections worldwide. M1T1 was among few GAS strains that have outnumbered other serotypes since the mid-1980s, and their increase in GAS populations coincided with the resurgence of invasive and severe forms of streptococcal diseases, especially the streptococcal toxic shock syndrome (STSS) and necrotizing fasciitis (NF). The resurgence of the severe diseases was thus suspected to be a result of an unusual pathogenic potential inherent to the few reemerging strains. This book, based on my PhD dissertation, discusses unique features that may have endowed the global M1T1 clone with its unusual properties, and the genomic and evolutionary basis for its emergence and persistence. Among M1T1''s unique features are its ability to switch to a hypervirulent phenotype with a survival advantage in deep tissues; the acquisition and exchange of mosaic prophages that encode novel exotoxins; and the production of the potent DNase Sda1, which frees the bacteria from the leukocyte extracellular traps.