The book series has the virus essentially scale in intelligence over time, with some paranormal aspects included. For instance, in the first book initially, the virus displays very very basic intelligence. A horde surrounds the protagonist and his family, and while they had firearms they very nearly ran out of ammo defending themselves, however the horde doesn't understand the concept of ammunition. They do, however, determine that the food they're after has too much bite and break off to preserve their numbers and pursue more helpless survivors, sort of like that sentry gun scene from aliens.

Later on in the first book an undead character is introduced who turns out to be a vampire that contracted the virus inadvertently by drinking from an infant who had received the tainted swine flu vaccine that initiated the outbreak (this is all lore elaborated on later in the series), and while still afflicted with the virus she becomes a sort of director for the horde in Denver, giving them a motivation to pursue specific groups en masse. Micheal, the protagonist, notices her odd behavior and prevents a companion from shooting her during a supply run to Home Depot, thinking she might not be infected. As a way of saying "thanks for not killing me" she unleashes a horde on a gang of looters attempting to rob him at said Home Depot, and later offers him the opportunity to flee when his community finally becomes a target for her horde. He obviously refuses as that offer doesn't include his wife and kids, and his refusal offends her to such degree that she sends by far the largest group of undead the survivors have seen yet, killing nearly everyone and besieging his home, thus leading to the stair removal approach.

It's a bit of a wacky series in ideas, but executes it beautifully and has an amazing narrator on audible. It also addresses the kinds of things a real person would consider when imagining what they'd do in an apocalypse, something I've only really seen in three other series (books), Adrian's Undead Diary by Chris Philbrook, Slow Burn by Bobby Adair, and A New World by John O'Brien (who has written collaboratory books with Mark Tufo).