https://phys.org/news/2018-06-oldest-bubonic-plague-genome-decoded.html

Recent studies of ancient Y. pestis genomes identified its earliest known variants, dating to the Late Neolithic and Early Bronze Age, but these genomes did not show the genetic signatures thought to make the plague particularly efficient—namely, adaptation to survival in fleas, which act as the main vectors that transmit the disease to mammals. This study aimed to look at more Bronze Age Y. pestis genomes, in order to investigate when and where these important adaptations occurred.

3,800-year-old double burial of two plague victims yields oldest bubonic plague genome to date