Compare the damage done when good-sized airplane crashes from an accident (typically 50 to 250 deaths), to the damage done when that same airplane is smashed into the WTC. (Estimate 1/2 of the approx. 5500 assumed dead = 2750 deaths).
Ten terrorists infected with smallpox could make it their one last goal in life to wander among crowds, coughing, sneezing, spreading mucus on doorknobs, and so forth. That would INITIALLY infect far more than the one 'index' case cited in the article. And some of those so-called 'first-wave' cases will board trains and planes, spreading smallpox from coast to coast, BEFORE THE FIRST CONFIRMED CASE IS EVEN IDENTIFIED.
We must compare apples to apples. Terrorist incidents are not 'unfortunate'. They are not 'tragic'. They are evil, and deliberate.