Question: What is definition of "airborne?"

[Aaron Delwiche]

In discussions about Ebola-Zaire and Ebola-Reston, people often dispute
whether or not the viruses are airborne.  What does it mean for a virus to
be airborne?  Is an airborne virus one that is transmitted via coughing on
someone's face?  Or does an airborne virus float through the air like
pollen?

When the World Health organization refers to a virus as "airborne," what 
exactly do they mean?

I realize that Ebola-Zaire is not considered to be an airborne virus.

Aaron Delwiche
redwood at halcyon.com