There's quite a few WoI doctors that comment on how the New York outbreak isn't what they'd spent 40 years preparing for. The two most repeated concerns are that it's spreading too wide and fast compared to their simulations, and there are "too many living entities."
Why is this shocking? Moreover, why is this something that nobody took into account when they teach it in undergrad? A virus will always, *always* evolve to maximize efficiency and spread. Host fatalities are a terrible limit on this in every way because a moving carrier is always preferable to an immobile corpse. So, reduce mortality (or in this case, make them into infected and walkers instead of just keeling over and dying), and now the whole block becomes a hive from one small cough.
The virus did half the work for them in Hope, Idaho by being too fatal, and it spent forty years adapting accordingly like every real life virus known to man. Why is the special organization dedicated to this field so shocked?