We introduce the alternate social networks (ASN) model to study the epidemic threshold of SIS epidemic. The alternate social networks consist of a family network and a public network, mimicking the human contacts during some time (nighttime) and other time (daytime) respectively. Both the family network and the public network are constructed by a set of sub networks which can exhibit small-world properties, scale-free degree distribution or the household structure, representing various types of local interactions among social groups in modern society. Simulations show that the ASN has the essential characteristics of social networks, and the local fully connected structures (households) as well as the existence of local structures (publics) in the public network are two dominating ingredients for the epidemic threshold. Moreover, results show that the epidemic threshold in ASN is independent of the initial condition and the system size.

PACS numbers: 89.75.Fb, 89.75.Hc, 87.23.Ge, 87.19.Xx