  The Internet, as well as many other networks, has a very complex connectivity
recently modeled by the class of scale-free networks. This feature, which
appears to be very efficient for a communications network, favors at the same
time the spreading of computer viruses. We analyze real data from computer
virus infections and find the average lifetime and prevalence of viral strains
on the Internet. We define a dynamical model for the spreading of infections on
scale-free networks, finding the absence of an epidemic threshold and its
associated critical behavior. This new epidemiological framework rationalize
data of computer viruses and could help in the understanding of other spreading
phenomena on communication and social networks.
