We report the results of a numerical investigation on networks of van der Pol-Fitzhugh-Nagumo oscillators. The system models the features of an excitable medium in which each of the elements can be in a silent, active or oscillatory regime. We argue that if the overall medium is, for example, in a silent state and close to threshold, then a (very) small percentage of diverse, active elements is sufficient to drive the complete system into an oscillatory phase. The emerging oscillations show a considerable amount of synchronisation and the phenomenon is seen to persist even when dynamical noise is introduced.