Wireless Body Area Network (WBAN) is a time critical remote network where various physiological information of patients are sent through satisfying stringent QoS requirements. However, performance of WBAN may degrade severely due to the coexistence problem, a well known phenomenon where different wireless channel access technologies share the overlapped frequency bands. In this paper, the coexistence problem of WBAN is addressed in terms of mutual interference mitigation and cross interference mitigation. A fuzzy logic based algorithm is proposed to detect the mutual interference and to mitigate it. Furthermore, a channel hopping technique is investigated to mitigate the cross interference problem. Simulation results show that our proposed techniques effectively mitigate the coexistence problem in WBAN scenarios.