Network reliability is very important in wireless body area network since the vital human life might be jeopardized, unless managed properly. In this article, a new modeling and analysis of node behaviors in wireless body area networks is presented for increasing reliability in the presence of multi-type failures, while saving energy. First, the nodes are classified into types with regard to their capabilities on relaying and sensing. Then, the node behaviors in the presence of failures such as energy exhaustion and/or malicious attacks have been modeled using a novel semi-Markov process. Finally, simulation has been done to evaluate the performance.