Microbiology 101

Lesson #1: Respect all bacterial cultures.

Doesn't matter if it's the lowly E. coli K-12, or Yersinia pestis. If you treat them all the same, respect them, and handle them with utmost care, you'll be alright. Get lax and lazy, and bad things can happen. Labs have safety procedures and precautions for a reason.

Attenuated doesn't necessarily mean it won't cause disease. It appears this researcher may have found that out the hard way.