re: Dallas Ebola patient to be prosecuted, circle of contact increased to 100

You are citing a generic number originating from a paper that explored the feasibility of certain classes of agents (in this case viral heamorragic fevers) as biological weapons.

There is little hard evidence to pin it down, but a number of those who work in the field with it think that number is absurd. It's "low," but it's low compared to some pathogen need 10000+ to cause a disease

And even if you want to take that number as a given, you have to consider that that means:

That is the number of organisms that gets in your blood somehow, not what you are exposed to. If you swallow fluid with a 1000 Ebola virions and you don't have a cut, gingivitis, or an ulcer, you won't come down with it.