Epidemic models have been a widely used mathematical tool in network security and social networks to study malware propagation and information dissemination. However, the relationships and the differences of discrete-time and continuous-time epidemic models in networks have not been systematically studied yet. In this paper, we focus on the susceptible-infectious model and attempt to connect and compare different discrete-time and continuous-time epidemic models through both theoretical analysis and empirical verification. We find that epidemic models can be distinguished based on whether a model considers the following three key factors: time intervals, spatial dependence among nodes, and linearization. We theoretically and empirically show that ignoring time intervals, assuming spatial independence among nodes, or applying linearization can cause a model to possibly over-predict the propagation speed of an epidemic. Especially, we discover that a widely used continuous-time epidemic model cannot accurately characterize the spread of the actual epidemic by ignoring both time intervals and spatial dependence among nodes. INDEX TERMS Epidemic models, susceptible-infectious (SI) model, discrete-time epidemic models, continuous-time epidemic models. ZESHENG CHEN (S'03-M'07-SM'16) received the M.S. and Ph.D. degrees from the Georgia Institute of Technology, in 2005 and 2007, respectively. He is currently an Assistant Professor with the Department of Computer Science, Purdue University Fort Wayne. His current research interests include network security, the Internet of Things, social networks, and performance evaluation of communication networks.