Law of the Jungle — Franklin W. Dixon

THE KILLING GAME
Frank and Joe Hardy are bound for the jungles of Borneo, and the journey may be one of the most desperate and dangerous of their lives. Following a cryptic map sent to them by their father, the boys head into the heart of darkness, where they make a terrifying discovery: Fenton Hardy is on the brink of death!

Someone has injected Fenton with a fatal dose of anthrax-B, and the Hardys have only seven days to find the cure...or lose their father forever. The news gets worse when they learn who and what they are up against: the Assassins. Experts in terror, the Assassins are the epitome of evil -- an international network that lives to kill!