What is the modern black death?

Today's strain. If i understand your question correctly, you are asking about different strains of bubonic plague. People have looked at the genomes of the plague bacillus from today and compared it to the bacilli left in remains of victims of the epidemic in India in the late 1800's, the medieval Black Plague, and that in the ancient world and traced its molecular evolution; all 3 strains still occur in nature.