Maybe not a true bio-weapon,
but something along the lines of what our goverment did in the 1950's when it released "nonfatal" diseases into certain urban centers to see how a true bio-weapon would spread. In the course of those tests, there were people that died from the pathogens. Nothing close to the number of deaths from SARS, but biotech's come a long way since then, and global travel has mushroomed has well.