Flu by Wayne Simmons opens with a quarantine team going to a council flat. Their goal is to seal in the newest of the flu victims before they can spread the infection further. This outbreak makes the outbreak after WWI and the recent swine/avian flu thing look like picnics.
They find what they expect — a recently deceased person — but then things go horribly wrong. The body re-animates. The flu has taken on a new form, one that kills and then creates a zombie from the corpse. Zombie bites can bring on flu symptoms.
The remainder of the book is divided among different sets of survivors as they try to avoid the zombies, stay well and find food. It's a fairly typical post-BIG EVENT survivalist novel at this point. The only difference is its setting, Belfast.
The book ends with a brief hint of what caused this outbreak. More information about its origins would have sealed the deal for me. There is also hint of things to come — which presumably are addressed in further detail in the sequel, Fever (2012).