In an effort to understand the molecular genetic basis of temporal variation in 
frequency and severity of bacterial disease, genetic relationships among strains 
of Streptococcus pyogenes that caused scarlet fever epidemics in Canada in the 
early 1940s and in eastern Germany in the 1960s to 1980s were studied. 
Application of multilocus enzyme electrophoresis and comparative sequencing of 
the gene (speA) encoding streptococcal pyrogenic exotoxin A (scarlet fever 
toxin) revealed that new waves of scarlet fever are associated with an increase 
in frequency of S. pyogenes clones carrying variant speA alleles. This finding 
suggests that the occurrence of new scarlet fever epidemics can be predicted by 
comprehensive monitoring of the frequency of S. pyogenes clones with variant 
toxin alleles.