Is Africa under threat from Islamist extremists?

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Many see Africa as a breeding ground for violent Islamist extremism. But is this true? And if it is, how big is the threat? (Photo: Nigerian army forces retake the north-eastern town of Gwoza, believed to be the headquarters of militant Islamist group Boko Haram, 27 March 2015. Credit: Tomi Oladipo, BBC)