A Sudden Outbreak of Magic - Contagious Magic, #1

Magic: more contagious than the common cold.Kelley Strickland and her twin brother Jeroan accidentally step into an ancient battle between a power-hungry Sorcerer-who wants to rid the world of what he calls "renegade" magic-users-and a good Sorcerer who can barely remember his own name. Kelley soon learns that her new-found way of using magic just might save her adopted city, her brother, and her new, magically infected friends.
Or it could doom them all. A fantasy novel for all ages about ancient power, growing up, and finding your place within those two very different realms.  The first book in the Contagious Magic series, followed by A Wild Epidemic of Magic.