In a frequency hopping multiple access (FHMA) system a set of users communicates simultaneously using frequency hopping sequences defined on the same set of frequency channels. A frequency hopping sequence is comprised of frequency channels used to switch channels as communication progresses. Much of the research on the performance of FHMA systems is based on either pairwise mutual interference or adver-sarial interference but not both. In this paper, we evaluate the performance of an FHMA system with respect to both group-wise mutual interference and adversarial interference (jamming), bearing in mind that more than two users may be transmitting simultaneously in the presence of an adversary. Our main contributions are as follows. We point out a correspondence between a cover-free code and a frequency hopping scheme. Cover-free codes give a well defined structure on the transmission capacity of a frequency hopping multiple access system. Further, we specify a jammer model for an FHMA. Considering the resources and knowledge of a jammer, we look at the possible mitigation strategies to be employed by a frequency hopping scheme against the jammer, and determine necessary and desirable additional properties of cover-free codes such that they may be used in the presence of adversarial interference.