Pervasive healthcare is one of the most important applications of the Internet of Things (IoT). As part of the IoT, the Wireless Sensor Networks (WSNs) are responsible for sensing the abnormal behavior of the elderly or patients. In this paper, the authors design and implement a fall detection system called SensFall. With the resource restricted sensor nodes, it is vital to find an efficient feature to describe the scene. Based on the optical flow analysis, it can be observed that the thermal energy variation of each sub-region of the monitored region is a salient spatio-temporal feature that characterizes the fall.