“…The SIS epidemic model is one of the most basic epidemic models, and assumes that recovered individuals become susceptible again immediately without immunity (see, e.g., Diekmann and Heesterbeek [8]). Many kinds of SIS epidemic models have been studied by researchers (see, e.g., [26,4,12,28,20,11,33,2,37,10,38,31,3,13,29,24,25]).…”