... And most of the infected die so rapidly?
I mean, they literally go over this argument at one point of the movie.
The one reason why the corona virus spread is so effective is because you can be contagious, without having any symptoms, for up to 12 days.
Well, it would just need to behave like Corona does: The infected are extremely infectious and mostly asymptomatic during the incubation period. In that case, a virus will spread very quickly.
Once the infection has spread, it wouldn't matter if the previous infected die quickly, because it already has multiple new asymptomatic spreaders.
Also known as having a large R0:
https://en.wikipedia.org/wiki/Basic_reproduction_number
R0 of 3 means each infected will spread the virus to 3 new targets.
It would need to behave like Corona does, with the incubation. But in the movie it doesn't, which I think is the OP's point.
How could it spread according to their projections based on how we know it behaves from what we see on screen?
My opinion: it's not that kind of movie, kid.