You do make good arguments. The fall of PB did sound a bit contrived in canon and those Project members living there should not be dumb.
As for a disease that can kill 100% they might not be impossible, but likely difficult to pull off. A research project on aerosolized prions was able to get 100% fatality rates in mice.
http://www.upmc-cbn.org/report_archi..._01212011.html
This does not mean it can happen in humans, but it is a worrying study. Infection this way would probably not be person to person through the air, but some aerosol dispersal device could infect a crowd.