- The M protein encoded by the emm gene is a major virulence factor of GAS. M1, M2, M3, M4, M6, and M22 strains have been shown to be associated with outbreaks of scarlet fever; 1e4 M1 and M3 were associated with invasive disease and fatal infections in Britain from 1980 to 1990 and in the United States from 1995 to 1999 M5, M6 and M18 was associated with acute rheumatic fever.
- Most deaths caused by GAS were because of invasive diseases. Since late 1980s, there has been a marked increase of STSS, which was associated with shock and multiple-organ failure.
- A new, highly virulent subclone of serotype M1T1, being studied to evolve as a result of diversification of the bacteria and acquisition of new genes, emerged to cause severe diseases.