http://www.chass.utoronto.ca/~wellman/publications/viruspaper/version.PDF

We’ve been discussing network diffusion in class recently – namely, how a trend propagates throughout a network. The paper above analyzes viruses, one such “trend.” It explores the effect of tightly- and loosely-knit networks on several types of viruses. Specifically, the paper compares biological viruses, computer viruses, and viral marketing.

The paper first identifies two extremes in network structure. Densely knit groups involve members who are in frequently contact with each other and infrequent contact with those outside the group. Ramified networks are just the opposite: fewer members are in contact with each other, and each member has a wider range of contacts.

Networks with densely knit groups tend to reinforce each other, so if a virus can infect even a single member of the group, it can quickly spread to the rest of the group. Biological viruses spread based physical contact; so if a family member catches a cold, the rest of the household might soon get sick. Moreover, groups often share certain characteristics that make them more susceptible to a particular virus. Business computer networks often have identical hardware and software configurations for each of the workstations in the building. A computer virus that takes advantage of a security flaw in one system can also compromise every other system in the network with the same tactic. In both biology and computing, viruses are more effective if they can “stick” to a host for longer. The paper refers to this as the “stickiness factor.” Viruses that kill a person or wipe a hard drive quickly have a low stickiness factor and cannot stay around long enough to spread well. In viral marketing, people in the same group often have similar tastes and interests. Thus, introducing a new product to a single member helps the rest of the group discover the product quickly. The entire group is likely to use the product shortly thereafter.

Ramified networks contain a number of weak ties that bridge different groups together. These “structural holes” are the means by which viruses can spread to a more heterogeneous population. This the means by which HIV was eventually able to spread outside the gay population into the general public. Modern computer viruses often spread by e-mailing a copy of themselves to everyone on a person’s contact list. This includes both a person’s strong and weak ties, enabling a virus to extend outside that person’s close circle of friends. Structural holes can be used with great success in viral marketing as well. Free e-mail services such as Yahoo! Mail and Hotmail tag a footer onto all outgoing e-mails, advertising the e-mail service to the recipients. In fact, the Internet virtually eliminates physical distance barriers to viral marketing. Blog posts, social networking sites, forums, and newsgroups can serve to explode the popularity of ideas, trends, and products.

In summary, most kinds of viruses spread similarly in particular types of networks. Densely-knit and ramified networks are two extreme cases; real social networks are always somewhere in between. The paper refers to these as “glocalized” networks. These types of networks spread rapidly within clusters, but more slowly across clusters.

The author of the paper is Barry Wellman, a professor at the University of Toronto. He studies network interactions and communications for both social and computer networks.