Based on what I saw in the movie, didn't read the book the rate of infection is unrealistic, at least the time it takes from initial exposure to full on aggress

Based on what I saw in the movie, didn't read the book the rate of infection is unrealistic, at least the time it takes from initial exposure to full on aggressive zombie mode was a few seconds. Realistically this would take much longer than that which means we won't end up with hordes of zombies instantly.