I am assuming that your question refers to the novel called Blackout by Grant. There are other novels called "Blackout," including one that is based on the TV series Buffy the Vampire Slayer. If your question refers to the latter, please repost your question.
There is a virus that turns any dead mammal over 40 pounds into a zombie. Interestingly enough, in this novel, it seems like the Center for Disease Control is actually the villain as it creates new strains of the virus to infect others.
Shaun and Rebecca and friends try to find out the reason why, but they are put in a trap and need a new identity first. They approach the strange character called the Monkey, who says that he will give them a new identity once they break into the Center for Disease Control in Seattle to investigate.
Meanwhile, the character named Georgia dies. She wakes up in the Center for Disease Control and finds out that she is a 97% clone. There are also other clones made of her that are more compliant. Georgia figures this out and begins killing the other clones of herself. Unfortunately, the group is taken to the White House to meet the president, who is also in on the conspiracy. The new strain is let loose in the White House, and Rebecca sacrifices herself to save others.
It turns out that the Center for Disease Control did create a cure for the disease, but the cure for the virus destroys the immune system. The Center for Disease Control was, therefore, creating new strains so as not to destroy the immunity of people. So, the conspiracy is revealed, the world is alerted, and Sean and Georgia escape publicity at the end as heroes.