We used the network data collected on the social network website Facebook (www.facebook.com) by Traud et al. [41] . The data contains the friendship network at five US universities, where nodes represent individuals (i.e. members of the university), and edges represent friendship links between two individuals. Additionally, the data includes covariate information (if available) about the individuals, such as the gender of the individual, the dormitory residence, major (field of specialization) etc. While such friendship network data are interesting for various reasons, they do not necessarily reflect the contact network relevant for the spread of infectious diseases. Clearly, a friendship connection between two individuals on a social network website does not necessarily mean that there is also a connection between the two individuals in the contact network relevant to the spread of infectious diseases.