In the book, the creation of the vampires sounds exactly like how zombies were created in various movies. There was some kind of crazy bacterial spores that became airborne and traveled within a dust storm. Anyone caught in the dust storm was bound to get a few tiny scratches, and once that happened, the bacteria attacked the wound and infected their body. If you were attacked by one of the infected, chances are you'd turn into one of them. In the book, Neville makes a distinction between "dead" vampires and "live" vampires once he realizes bullets kill some of them, but not others. You fit into one of the groups depending on how you were originally infected. The author spent a lot of time sending Neville to the library and going into detail about how this might be scientifically possible.
Honestly, the author could have wrote 10 pages of nothing, but it sounds more plausible than a cancer cure hulking out, and then somehow managing to become airborne.