Simplify the following text for a Grade 16 native English-speaking person.
Original: It was the culmination of research in the 1930s and early 1940s at the Rockefeller Institute for Medical Research to purify and characterize the `` transforming principle '' responsible for the transformation phenomenon first described in Griffith 's experiment of 1928 : killed Streptococcus pneumoniae of the virulent strain type III-S , when injected along with living but non-virulent type II-R pneumococci , resulted in a deadly infection of type III-S pneumococci .
Simplified: This was the culmination of research in the 1930s and early 1940s at the Rockefeller Institute for Medical Research to purify and characterize the `` transforming principle '' responsible for the phenomenon first described in Griffith 's experiment of 1928 . In this early experiment dead Streptococcus pneumoniae of the virulent strain type III-S , when injected along with living but non-virulent type II-R pneumococci , resulted in a deadly infection of type III-S pneumococci .