How might the Columbian Exchange be considered a bio-terrorist attack?
An interesting idea; as smallpox, etc. devastated native American populations, sometimes wiping out entire cultures. Jared Diamond makes this point quite well in Guns, Germs and Steel. One of the more ironic elements is that large numbers of people died who never came into contact with Europeans; the diseases were that pervasive.
A problem with considering the entire exchange as biological warfare is that by and large, Europeans did not intend to use germ warfare to destroy these people; it was an incidental and to a large extent unintended consequence. One should also consider the fact that the Indians returned the favor: syphilis was unknown in Europe until European sailors and explorers intermingled with the Indians; yet again, however, this was an incidental and unintended consequence. That being the case, I'm not sure "biological terrorism" is the proper term.
There was at least one incident in which germ warfare is unquestionably the term: settlers at Jamestown gave Indians blankets which they knew to be infected with smallpox in a deliberate attempt to spread the disease. Other than this one incident (when the poison they otherwise would have used wasn't available) I'm not sure the entire exchange qualifies.
The Columbian Exchange could be seen in this way because it included the introduction of germs to the New World. These germs caused huge levels of mortality among natives of the Americas because they had never been exposed to those germs before. Some scholars believe that as many as 90% of Native Americans were killed by diseases from the Old World. Others put the mortality at 50%. Regardless of which of these numbers is accurate, you can see why some might call this bioterrorism. By introducing their germs (albeit unknowingly, at first) to the New World, Europeans caused the deaths of huge numbers of people.