Well, first the good news. Blattner says that Y. pestis evolves relatively slowly. Unlike viruses that mutate constantly, bacteria are, in general, much more stable. Now the bad news: Bacteria do indeed pick up new genes, as we have seen. A strain of plague found in Madagascar in the late 1990s, for example, was resistant to five antibiotics. The bacillus apparently picked up the genes that gave it hardiness from another bacterial species. Yes, another huge, catastrophic outbreak is possible. But no one can predict when or where.