http://www.reddit.com/r/ebola/comments/2gu1mn/ebola_3200000_virions_per_milliliter_of_blood/
Nice link for quantifying the viron per droplet and what it means. Difficult to develop a simple metric.
http://nypost.com/2014/10/04/sick-passenger-investigated-for-ebola-at-newark-airport/
The last line is a hair-raiser.