Frederick griffith was one of the leading scientists looking at the genetic role of dna. he was working on two strains of streptococcus pneumoniae: the rough strain (r) did not cause pneumonia, but the smooth strain (s) did. when griffith heat-killed the s strain and gave both the heat-killed s strain and the r strain to mice, the mice developed pneumonia and died. what happened in this experiment that caused the mice to develop pneumonia? the r strain mutated and thus was able to cause pneumonia. there was more of the r strain added than previously. the immune systems of the mice were unable to cope with both the s and r strains simultaneously. even though the s strain had been heat-killed, it changed the r strain.