"Mister Monday: The Keys to the Kingdom, Book 1" by Garth Nix

Out-of-this-world magical adventure series for teens from the award-winning, New York Times bestselling author of Sabriel and the Old Kingdom series

Seven days. Seven keys. One very unlikely hero.

Arthur Penhaligon is a normal teenage boy, trying to fit in at school. But when he suffers a serious asthma attack, his life is unexpectedly saved by two strange characters who appear out of nowhere - and a key shaped like the minute hand of a clock.

Arthur's life may have been saved, but his world is in danger. An incurable epidemic is spreading throughout the country, and his foster mother, a scientist, is desperately trying to help find the cure. And it is not long before Arthur realizes that a chilling person named Mister Monday will stop at nothing to get the key back from Arthur...