How does Preston manage to evoke feelings of fear in the reader?

On Page 98, the reader is faced with the information that Ebola Sudan has a 50% fatality rate, similar to that of the Black Plague. The novel then speaks to what would have happened had the virus spread...... a complete catastrophe in light of the spped at which it would have permeated the world. This is shortly followed by the notation that this never happened..... but it could have.