text: M. J. Pettit, professor of clinical microbiology in the UCLA medical school, ended his lecture on plague and tularemia. As his students bustled and chattered, he prepared to depart.
  
  "Dr. Pettit, do you have a second? I have a question." The professor looked up. It was Marilyn Sturtevant, a studious blonde—one of the top achievers on exams.
  
  "Sure, What is it?"
  
  "Before today's lecture, I thought plague was a disease of the past like smallpox—wiped out. But you say it's still with us."
  
  "That's right."
  
  "You also said it's common in wild rodents in Southern California."
  
  Pettit nodded.
  
  The woman shook her head. "I guess I'm kind of shocked. I had no idea we had plague locally,"
  
  "Most people don't... not until there's a case reported in the news or a campground is closed because plague bacteria are found in squirrels or other animals. Do a web search. You'll be amazed."
  
  "I guess what I want to ask is, what is the probability of a black plague epidemic like that in Europe in the fourteenth century?"
  
  "Excellent question, Marilyn. The odds are nearly zero. Why? Because the virulent genetic strain of Yersinia pestis responsible for the black death is thought to be extinct."
  
  "Oohh. So there are different strains of plague bacteria that vary in their pathogenicity."
  
  "Exactly. Remember, however, that under the right conditions, even a relatively benign strain can become virulent."
  
  Sturtevant scribbled rapidly on her electronic pad. "Thank you, sir."
  
  Pettit smiled. "You're quite welcome... and please, drop the sir."
  
  "Right." She headed out the door, turned, and gave him a wave.